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2
3===========================
4Unreliable Guide To Locking
5===========================
6
7:Author: Rusty Russell
8
9Introduction
10============
11
12Welcome, to Rusty's Remarkably Unreliable Guide to Kernel Locking
13issues. This document describes the locking systems in the Linux Kernel
14in 2.6.
15
16With the wide availability of HyperThreading, and preemption in the
17Linux Kernel, everyone hacking on the kernel needs to know the
18fundamentals of concurrency and locking for SMP.
19
20The Problem With Concurrency
21============================
22
23(Skip this if you know what a Race Condition is).
24
25In a normal program, you can increment a counter like so:
26
27::
28
29 very_important_count++;
30
31
32This is what they would expect to happen:
33
34
35.. table:: Expected Results
36
37 +------------------------------------+------------------------------------+
38 | Instance 1 | Instance 2 |
39 +====================================+====================================+
40 | read very_important_count (5) | |
41 +------------------------------------+------------------------------------+
42 | add 1 (6) | |
43 +------------------------------------+------------------------------------+
44 | write very_important_count (6) | |
45 +------------------------------------+------------------------------------+
46 | | read very_important_count (6) |
47 +------------------------------------+------------------------------------+
48 | | add 1 (7) |
49 +------------------------------------+------------------------------------+
50 | | write very_important_count (7) |
51 +------------------------------------+------------------------------------+
52
53This is what might happen:
54
55.. table:: Possible Results
56
57 +------------------------------------+------------------------------------+
58 | Instance 1 | Instance 2 |
59 +====================================+====================================+
60 | read very_important_count (5) | |
61 +------------------------------------+------------------------------------+
62 | | read very_important_count (5) |
63 +------------------------------------+------------------------------------+
64 | add 1 (6) | |
65 +------------------------------------+------------------------------------+
66 | | add 1 (6) |
67 +------------------------------------+------------------------------------+
68 | write very_important_count (6) | |
69 +------------------------------------+------------------------------------+
70 | | write very_important_count (6) |
71 +------------------------------------+------------------------------------+
72
73
74Race Conditions and Critical Regions
75------------------------------------
76
77This overlap, where the result depends on the relative timing of
78multiple tasks, is called a race condition. The piece of code containing
79the concurrency issue is called a critical region. And especially since
80Linux starting running on SMP machines, they became one of the major
81issues in kernel design and implementation.
82
83Preemption can have the same effect, even if there is only one CPU: by
84preempting one task during the critical region, we have exactly the same
85race condition. In this case the thread which preempts might run the
86critical region itself.
87
88The solution is to recognize when these simultaneous accesses occur, and
89use locks to make sure that only one instance can enter the critical
90region at any time. There are many friendly primitives in the Linux
91kernel to help you do this. And then there are the unfriendly
92primitives, but I'll pretend they don't exist.
93
94Locking in the Linux Kernel
95===========================
96
97If I could give you one piece of advice on locking: **keep it simple**.
98
99Be reluctant to introduce new locks.
100
101Two Main Types of Kernel Locks: Spinlocks and Mutexes
102-----------------------------------------------------
103
104There are two main types of kernel locks. The fundamental type is the
105spinlock (``include/asm/spinlock.h``), which is a very simple
106single-holder lock: if you can't get the spinlock, you keep trying
107(spinning) until you can. Spinlocks are very small and fast, and can be
108used anywhere.
109
110The second type is a mutex (``include/linux/mutex.h``): it is like a
111spinlock, but you may block holding a mutex. If you can't lock a mutex,
112your task will suspend itself, and be woken up when the mutex is
113released. This means the CPU can do something else while you are
114waiting. There are many cases when you simply can't sleep (see
115`What Functions Are Safe To Call From Interrupts?`_),
116and so have to use a spinlock instead.
117
118Neither type of lock is recursive: see
119`Deadlock: Simple and Advanced`_.
120
121Locks and Uniprocessor Kernels
122------------------------------
123
124For kernels compiled without ``CONFIG_SMP``, and without
125``CONFIG_PREEMPT`` spinlocks do not exist at all. This is an excellent
126design decision: when no-one else can run at the same time, there is no
127reason to have a lock.
128
129If the kernel is compiled without ``CONFIG_SMP``, but ``CONFIG_PREEMPT``
130is set, then spinlocks simply disable preemption, which is sufficient to
131prevent any races. For most purposes, we can think of preemption as
132equivalent to SMP, and not worry about it separately.
133
134You should always test your locking code with ``CONFIG_SMP`` and
135``CONFIG_PREEMPT`` enabled, even if you don't have an SMP test box,
136because it will still catch some kinds of locking bugs.
137
138Mutexes still exist, because they are required for synchronization
139between user contexts, as we will see below.
140
141Locking Only In User Context
142----------------------------
143
144If you have a data structure which is only ever accessed from user
145context, then you can use a simple mutex (``include/linux/mutex.h``) to
146protect it. This is the most trivial case: you initialize the mutex.
147Then you can call mutex_lock_interruptible() to grab the
148mutex, and mutex_unlock() to release it. There is also a
149mutex_lock(), which should be avoided, because it will
150not return if a signal is received.
151
152Example: ``net/netfilter/nf_sockopt.c`` allows registration of new
153setsockopt() and getsockopt() calls, with
154nf_register_sockopt(). Registration and de-registration
155are only done on module load and unload (and boot time, where there is
156no concurrency), and the list of registrations is only consulted for an
157unknown setsockopt() or getsockopt() system
158call. The ``nf_sockopt_mutex`` is perfect to protect this, especially
159since the setsockopt and getsockopt calls may well sleep.
160
161Locking Between User Context and Softirqs
162-----------------------------------------
163
164If a softirq shares data with user context, you have two problems.
165Firstly, the current user context can be interrupted by a softirq, and
166secondly, the critical region could be entered from another CPU. This is
167where spin_lock_bh() (``include/linux/spinlock.h``) is
168used. It disables softirqs on that CPU, then grabs the lock.
169spin_unlock_bh() does the reverse. (The '_bh' suffix is
170a historical reference to "Bottom Halves", the old name for software
171interrupts. It should really be called spin_lock_softirq()' in a
172perfect world).
173
174Note that you can also use spin_lock_irq() or
175spin_lock_irqsave() here, which stop hardware interrupts
176as well: see `Hard IRQ Context`_.
177
178This works perfectly for UP as well: the spin lock vanishes, and this
179macro simply becomes local_bh_disable()
180(``include/linux/interrupt.h``), which protects you from the softirq
181being run.
182
183Locking Between User Context and Tasklets
184-----------------------------------------
185
186This is exactly the same as above, because tasklets are actually run
187from a softirq.
188
189Locking Between User Context and Timers
190---------------------------------------
191
192This, too, is exactly the same as above, because timers are actually run
193from a softirq. From a locking point of view, tasklets and timers are
194identical.
195
196Locking Between Tasklets/Timers
197-------------------------------
198
199Sometimes a tasklet or timer might want to share data with another
200tasklet or timer.
201
202The Same Tasklet/Timer
203~~~~~~~~~~~~~~~~~~~~~~
204
205Since a tasklet is never run on two CPUs at once, you don't need to
206worry about your tasklet being reentrant (running twice at once), even
207on SMP.
208
209Different Tasklets/Timers
210~~~~~~~~~~~~~~~~~~~~~~~~~
211
212If another tasklet/timer wants to share data with your tasklet or timer
213, you will both need to use spin_lock() and
214spin_unlock() calls. spin_lock_bh() is
215unnecessary here, as you are already in a tasklet, and none will be run
216on the same CPU.
217
218Locking Between Softirqs
219------------------------
220
221Often a softirq might want to share data with itself or a tasklet/timer.
222
223The Same Softirq
224~~~~~~~~~~~~~~~~
225
226The same softirq can run on the other CPUs: you can use a per-CPU array
227(see `Per-CPU Data`_) for better performance. If you're
228going so far as to use a softirq, you probably care about scalable
229performance enough to justify the extra complexity.
230
231You'll need to use spin_lock() and
232spin_unlock() for shared data.
233
234Different Softirqs
235~~~~~~~~~~~~~~~~~~
236
237You'll need to use spin_lock() and
238spin_unlock() for shared data, whether it be a timer,
239tasklet, different softirq or the same or another softirq: any of them
240could be running on a different CPU.
241
242Hard IRQ Context
243================
244
245Hardware interrupts usually communicate with a tasklet or softirq.
246Frequently this involves putting work in a queue, which the softirq will
247take out.
248
249Locking Between Hard IRQ and Softirqs/Tasklets
250----------------------------------------------
251
252If a hardware irq handler shares data with a softirq, you have two
253concerns. Firstly, the softirq processing can be interrupted by a
254hardware interrupt, and secondly, the critical region could be entered
255by a hardware interrupt on another CPU. This is where
256spin_lock_irq() is used. It is defined to disable
257interrupts on that cpu, then grab the lock.
258spin_unlock_irq() does the reverse.
259
260The irq handler does not need to use spin_lock_irq(), because
261the softirq cannot run while the irq handler is running: it can use
262spin_lock(), which is slightly faster. The only exception
263would be if a different hardware irq handler uses the same lock:
264spin_lock_irq() will stop that from interrupting us.
265
266This works perfectly for UP as well: the spin lock vanishes, and this
267macro simply becomes local_irq_disable()
268(``include/asm/smp.h``), which protects you from the softirq/tasklet/BH
269being run.
270
271spin_lock_irqsave() (``include/linux/spinlock.h``) is a
272variant which saves whether interrupts were on or off in a flags word,
273which is passed to spin_unlock_irqrestore(). This means
274that the same code can be used inside an hard irq handler (where
275interrupts are already off) and in softirqs (where the irq disabling is
276required).
277
278Note that softirqs (and hence tasklets and timers) are run on return
279from hardware interrupts, so spin_lock_irq() also stops
280these. In that sense, spin_lock_irqsave() is the most
281general and powerful locking function.
282
283Locking Between Two Hard IRQ Handlers
284-------------------------------------
285
286It is rare to have to share data between two IRQ handlers, but if you
287do, spin_lock_irqsave() should be used: it is
288architecture-specific whether all interrupts are disabled inside irq
289handlers themselves.
290
291Cheat Sheet For Locking
292=======================
293
294Pete Zaitcev gives the following summary:
295
296- If you are in a process context (any syscall) and want to lock other
297 process out, use a mutex. You can take a mutex and sleep
298 (``copy_from_user()`` or ``kmalloc(x,GFP_KERNEL)``).
299
300- Otherwise (== data can be touched in an interrupt), use
301 spin_lock_irqsave() and
302 spin_unlock_irqrestore().
303
304- Avoid holding spinlock for more than 5 lines of code and across any
305 function call (except accessors like readb()).
306
307Table of Minimum Requirements
308-----------------------------
309
310The following table lists the **minimum** locking requirements between
311various contexts. In some cases, the same context can only be running on
312one CPU at a time, so no locking is required for that context (eg. a
313particular thread can only run on one CPU at a time, but if it needs
314shares data with another thread, locking is required).
315
316Remember the advice above: you can always use
317spin_lock_irqsave(), which is a superset of all other
318spinlock primitives.
319
320============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ==============
321. IRQ Handler A IRQ Handler B Softirq A Softirq B Tasklet A Tasklet B Timer A Timer B User Context A User Context B
322============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ==============
323IRQ Handler A None
324IRQ Handler B SLIS None
325Softirq A SLI SLI SL
326Softirq B SLI SLI SL SL
327Tasklet A SLI SLI SL SL None
328Tasklet B SLI SLI SL SL SL None
329Timer A SLI SLI SL SL SL SL None
330Timer B SLI SLI SL SL SL SL SL None
331User Context A SLI SLI SLBH SLBH SLBH SLBH SLBH SLBH None
332User Context B SLI SLI SLBH SLBH SLBH SLBH SLBH SLBH MLI None
333============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ==============
334
335Table: Table of Locking Requirements
336
337+--------+----------------------------+
338| SLIS | spin_lock_irqsave |
339+--------+----------------------------+
340| SLI | spin_lock_irq |
341+--------+----------------------------+
342| SL | spin_lock |
343+--------+----------------------------+
344| SLBH | spin_lock_bh |
345+--------+----------------------------+
346| MLI | mutex_lock_interruptible |
347+--------+----------------------------+
348
349Table: Legend for Locking Requirements Table
350
351The trylock Functions
352=====================
353
354There are functions that try to acquire a lock only once and immediately
355return a value telling about success or failure to acquire the lock.
356They can be used if you need no access to the data protected with the
357lock when some other thread is holding the lock. You should acquire the
358lock later if you then need access to the data protected with the lock.
359
360spin_trylock() does not spin but returns non-zero if it
361acquires the spinlock on the first try or 0 if not. This function can be
362used in all contexts like spin_lock(): you must have
363disabled the contexts that might interrupt you and acquire the spin
364lock.
365
366mutex_trylock() does not suspend your task but returns
367non-zero if it could lock the mutex on the first try or 0 if not. This
368function cannot be safely used in hardware or software interrupt
369contexts despite not sleeping.
370
371Common Examples
372===============
373
374Let's step through a simple example: a cache of number to name mappings.
375The cache keeps a count of how often each of the objects is used, and
376when it gets full, throws out the least used one.
377
378All In User Context
379-------------------
380
381For our first example, we assume that all operations are in user context
382(ie. from system calls), so we can sleep. This means we can use a mutex
383to protect the cache and all the objects within it. Here's the code::
384
385 #include <linux/list.h>
386 #include <linux/slab.h>
387 #include <linux/string.h>
388 #include <linux/mutex.h>
389 #include <asm/errno.h>
390
391 struct object
392 {
393 struct list_head list;
394 int id;
395 char name[32];
396 int popularity;
397 };
398
399 /* Protects the cache, cache_num, and the objects within it */
400 static DEFINE_MUTEX(cache_lock);
401 static LIST_HEAD(cache);
402 static unsigned int cache_num = 0;
403 #define MAX_CACHE_SIZE 10
404
405 /* Must be holding cache_lock */
406 static struct object *__cache_find(int id)
407 {
408 struct object *i;
409
410 list_for_each_entry(i, &cache, list)
411 if (i->id == id) {
412 i->popularity++;
413 return i;
414 }
415 return NULL;
416 }
417
418 /* Must be holding cache_lock */
419 static void __cache_delete(struct object *obj)
420 {
421 BUG_ON(!obj);
422 list_del(&obj->list);
423 kfree(obj);
424 cache_num--;
425 }
426
427 /* Must be holding cache_lock */
428 static void __cache_add(struct object *obj)
429 {
430 list_add(&obj->list, &cache);
431 if (++cache_num > MAX_CACHE_SIZE) {
432 struct object *i, *outcast = NULL;
433 list_for_each_entry(i, &cache, list) {
434 if (!outcast || i->popularity < outcast->popularity)
435 outcast = i;
436 }
437 __cache_delete(outcast);
438 }
439 }
440
441 int cache_add(int id, const char *name)
442 {
443 struct object *obj;
444
445 if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL)
446 return -ENOMEM;
447
448 strscpy(obj->name, name, sizeof(obj->name));
449 obj->id = id;
450 obj->popularity = 0;
451
452 mutex_lock(&cache_lock);
453 __cache_add(obj);
454 mutex_unlock(&cache_lock);
455 return 0;
456 }
457
458 void cache_delete(int id)
459 {
460 mutex_lock(&cache_lock);
461 __cache_delete(__cache_find(id));
462 mutex_unlock(&cache_lock);
463 }
464
465 int cache_find(int id, char *name)
466 {
467 struct object *obj;
468 int ret = -ENOENT;
469
470 mutex_lock(&cache_lock);
471 obj = __cache_find(id);
472 if (obj) {
473 ret = 0;
474 strcpy(name, obj->name);
475 }
476 mutex_unlock(&cache_lock);
477 return ret;
478 }
479
480Note that we always make sure we have the cache_lock when we add,
481delete, or look up the cache: both the cache infrastructure itself and
482the contents of the objects are protected by the lock. In this case it's
483easy, since we copy the data for the user, and never let them access the
484objects directly.
485
486There is a slight (and common) optimization here: in
487cache_add() we set up the fields of the object before
488grabbing the lock. This is safe, as no-one else can access it until we
489put it in cache.
490
491Accessing From Interrupt Context
492--------------------------------
493
494Now consider the case where cache_find() can be called
495from interrupt context: either a hardware interrupt or a softirq. An
496example would be a timer which deletes object from the cache.
497
498The change is shown below, in standard patch format: the ``-`` are lines
499which are taken away, and the ``+`` are lines which are added.
500
501::
502
503 --- cache.c.usercontext 2003-12-09 13:58:54.000000000 +1100
504 +++ cache.c.interrupt 2003-12-09 14:07:49.000000000 +1100
505 @@ -12,7 +12,7 @@
506 int popularity;
507 };
508
509 -static DEFINE_MUTEX(cache_lock);
510 +static DEFINE_SPINLOCK(cache_lock);
511 static LIST_HEAD(cache);
512 static unsigned int cache_num = 0;
513 #define MAX_CACHE_SIZE 10
514 @@ -55,6 +55,7 @@
515 int cache_add(int id, const char *name)
516 {
517 struct object *obj;
518 + unsigned long flags;
519
520 if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL)
521 return -ENOMEM;
522 @@ -63,30 +64,33 @@
523 obj->id = id;
524 obj->popularity = 0;
525
526 - mutex_lock(&cache_lock);
527 + spin_lock_irqsave(&cache_lock, flags);
528 __cache_add(obj);
529 - mutex_unlock(&cache_lock);
530 + spin_unlock_irqrestore(&cache_lock, flags);
531 return 0;
532 }
533
534 void cache_delete(int id)
535 {
536 - mutex_lock(&cache_lock);
537 + unsigned long flags;
538 +
539 + spin_lock_irqsave(&cache_lock, flags);
540 __cache_delete(__cache_find(id));
541 - mutex_unlock(&cache_lock);
542 + spin_unlock_irqrestore(&cache_lock, flags);
543 }
544
545 int cache_find(int id, char *name)
546 {
547 struct object *obj;
548 int ret = -ENOENT;
549 + unsigned long flags;
550
551 - mutex_lock(&cache_lock);
552 + spin_lock_irqsave(&cache_lock, flags);
553 obj = __cache_find(id);
554 if (obj) {
555 ret = 0;
556 strcpy(name, obj->name);
557 }
558 - mutex_unlock(&cache_lock);
559 + spin_unlock_irqrestore(&cache_lock, flags);
560 return ret;
561 }
562
563Note that the spin_lock_irqsave() will turn off
564interrupts if they are on, otherwise does nothing (if we are already in
565an interrupt handler), hence these functions are safe to call from any
566context.
567
568Unfortunately, cache_add() calls kmalloc()
569with the ``GFP_KERNEL`` flag, which is only legal in user context. I
570have assumed that cache_add() is still only called in
571user context, otherwise this should become a parameter to
572cache_add().
573
574Exposing Objects Outside This File
575----------------------------------
576
577If our objects contained more information, it might not be sufficient to
578copy the information in and out: other parts of the code might want to
579keep pointers to these objects, for example, rather than looking up the
580id every time. This produces two problems.
581
582The first problem is that we use the ``cache_lock`` to protect objects:
583we'd need to make this non-static so the rest of the code can use it.
584This makes locking trickier, as it is no longer all in one place.
585
586The second problem is the lifetime problem: if another structure keeps a
587pointer to an object, it presumably expects that pointer to remain
588valid. Unfortunately, this is only guaranteed while you hold the lock,
589otherwise someone might call cache_delete() and even
590worse, add another object, re-using the same address.
591
592As there is only one lock, you can't hold it forever: no-one else would
593get any work done.
594
595The solution to this problem is to use a reference count: everyone who
596has a pointer to the object increases it when they first get the object,
597and drops the reference count when they're finished with it. Whoever
598drops it to zero knows it is unused, and can actually delete it.
599
600Here is the code::
601
602 --- cache.c.interrupt 2003-12-09 14:25:43.000000000 +1100
603 +++ cache.c.refcnt 2003-12-09 14:33:05.000000000 +1100
604 @@ -7,6 +7,7 @@
605 struct object
606 {
607 struct list_head list;
608 + unsigned int refcnt;
609 int id;
610 char name[32];
611 int popularity;
612 @@ -17,6 +18,35 @@
613 static unsigned int cache_num = 0;
614 #define MAX_CACHE_SIZE 10
615
616 +static void __object_put(struct object *obj)
617 +{
618 + if (--obj->refcnt == 0)
619 + kfree(obj);
620 +}
621 +
622 +static void __object_get(struct object *obj)
623 +{
624 + obj->refcnt++;
625 +}
626 +
627 +void object_put(struct object *obj)
628 +{
629 + unsigned long flags;
630 +
631 + spin_lock_irqsave(&cache_lock, flags);
632 + __object_put(obj);
633 + spin_unlock_irqrestore(&cache_lock, flags);
634 +}
635 +
636 +void object_get(struct object *obj)
637 +{
638 + unsigned long flags;
639 +
640 + spin_lock_irqsave(&cache_lock, flags);
641 + __object_get(obj);
642 + spin_unlock_irqrestore(&cache_lock, flags);
643 +}
644 +
645 /* Must be holding cache_lock */
646 static struct object *__cache_find(int id)
647 {
648 @@ -35,6 +65,7 @@
649 {
650 BUG_ON(!obj);
651 list_del(&obj->list);
652 + __object_put(obj);
653 cache_num--;
654 }
655
656 @@ -63,6 +94,7 @@
657 strscpy(obj->name, name, sizeof(obj->name));
658 obj->id = id;
659 obj->popularity = 0;
660 + obj->refcnt = 1; /* The cache holds a reference */
661
662 spin_lock_irqsave(&cache_lock, flags);
663 __cache_add(obj);
664 @@ -79,18 +111,15 @@
665 spin_unlock_irqrestore(&cache_lock, flags);
666 }
667
668 -int cache_find(int id, char *name)
669 +struct object *cache_find(int id)
670 {
671 struct object *obj;
672 - int ret = -ENOENT;
673 unsigned long flags;
674
675 spin_lock_irqsave(&cache_lock, flags);
676 obj = __cache_find(id);
677 - if (obj) {
678 - ret = 0;
679 - strcpy(name, obj->name);
680 - }
681 + if (obj)
682 + __object_get(obj);
683 spin_unlock_irqrestore(&cache_lock, flags);
684 - return ret;
685 + return obj;
686 }
687
688We encapsulate the reference counting in the standard 'get' and 'put'
689functions. Now we can return the object itself from
690cache_find() which has the advantage that the user can
691now sleep holding the object (eg. to copy_to_user() to
692name to userspace).
693
694The other point to note is that I said a reference should be held for
695every pointer to the object: thus the reference count is 1 when first
696inserted into the cache. In some versions the framework does not hold a
697reference count, but they are more complicated.
698
699Using Atomic Operations For The Reference Count
700~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
701
702In practice, :c:type:`atomic_t` would usually be used for refcnt. There are a
703number of atomic operations defined in ``include/asm/atomic.h``: these
704are guaranteed to be seen atomically from all CPUs in the system, so no
705lock is required. In this case, it is simpler than using spinlocks,
706although for anything non-trivial using spinlocks is clearer. The
707atomic_inc() and atomic_dec_and_test()
708are used instead of the standard increment and decrement operators, and
709the lock is no longer used to protect the reference count itself.
710
711::
712
713 --- cache.c.refcnt 2003-12-09 15:00:35.000000000 +1100
714 +++ cache.c.refcnt-atomic 2003-12-11 15:49:42.000000000 +1100
715 @@ -7,7 +7,7 @@
716 struct object
717 {
718 struct list_head list;
719 - unsigned int refcnt;
720 + atomic_t refcnt;
721 int id;
722 char name[32];
723 int popularity;
724 @@ -18,33 +18,15 @@
725 static unsigned int cache_num = 0;
726 #define MAX_CACHE_SIZE 10
727
728 -static void __object_put(struct object *obj)
729 -{
730 - if (--obj->refcnt == 0)
731 - kfree(obj);
732 -}
733 -
734 -static void __object_get(struct object *obj)
735 -{
736 - obj->refcnt++;
737 -}
738 -
739 void object_put(struct object *obj)
740 {
741 - unsigned long flags;
742 -
743 - spin_lock_irqsave(&cache_lock, flags);
744 - __object_put(obj);
745 - spin_unlock_irqrestore(&cache_lock, flags);
746 + if (atomic_dec_and_test(&obj->refcnt))
747 + kfree(obj);
748 }
749
750 void object_get(struct object *obj)
751 {
752 - unsigned long flags;
753 -
754 - spin_lock_irqsave(&cache_lock, flags);
755 - __object_get(obj);
756 - spin_unlock_irqrestore(&cache_lock, flags);
757 + atomic_inc(&obj->refcnt);
758 }
759
760 /* Must be holding cache_lock */
761 @@ -65,7 +47,7 @@
762 {
763 BUG_ON(!obj);
764 list_del(&obj->list);
765 - __object_put(obj);
766 + object_put(obj);
767 cache_num--;
768 }
769
770 @@ -94,7 +76,7 @@
771 strscpy(obj->name, name, sizeof(obj->name));
772 obj->id = id;
773 obj->popularity = 0;
774 - obj->refcnt = 1; /* The cache holds a reference */
775 + atomic_set(&obj->refcnt, 1); /* The cache holds a reference */
776
777 spin_lock_irqsave(&cache_lock, flags);
778 __cache_add(obj);
779 @@ -119,7 +101,7 @@
780 spin_lock_irqsave(&cache_lock, flags);
781 obj = __cache_find(id);
782 if (obj)
783 - __object_get(obj);
784 + object_get(obj);
785 spin_unlock_irqrestore(&cache_lock, flags);
786 return obj;
787 }
788
789Protecting The Objects Themselves
790---------------------------------
791
792In these examples, we assumed that the objects (except the reference
793counts) never changed once they are created. If we wanted to allow the
794name to change, there are three possibilities:
795
796- You can make ``cache_lock`` non-static, and tell people to grab that
797 lock before changing the name in any object.
798
799- You can provide a cache_obj_rename() which grabs this
800 lock and changes the name for the caller, and tell everyone to use
801 that function.
802
803- You can make the ``cache_lock`` protect only the cache itself, and
804 use another lock to protect the name.
805
806Theoretically, you can make the locks as fine-grained as one lock for
807every field, for every object. In practice, the most common variants
808are:
809
810- One lock which protects the infrastructure (the ``cache`` list in
811 this example) and all the objects. This is what we have done so far.
812
813- One lock which protects the infrastructure (including the list
814 pointers inside the objects), and one lock inside the object which
815 protects the rest of that object.
816
817- Multiple locks to protect the infrastructure (eg. one lock per hash
818 chain), possibly with a separate per-object lock.
819
820Here is the "lock-per-object" implementation:
821
822::
823
824 --- cache.c.refcnt-atomic 2003-12-11 15:50:54.000000000 +1100
825 +++ cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100
826 @@ -6,11 +6,17 @@
827
828 struct object
829 {
830 + /* These two protected by cache_lock. */
831 struct list_head list;
832 + int popularity;
833 +
834 atomic_t refcnt;
835 +
836 + /* Doesn't change once created. */
837 int id;
838 +
839 + spinlock_t lock; /* Protects the name */
840 char name[32];
841 - int popularity;
842 };
843
844 static DEFINE_SPINLOCK(cache_lock);
845 @@ -77,6 +84,7 @@
846 obj->id = id;
847 obj->popularity = 0;
848 atomic_set(&obj->refcnt, 1); /* The cache holds a reference */
849 + spin_lock_init(&obj->lock);
850
851 spin_lock_irqsave(&cache_lock, flags);
852 __cache_add(obj);
853
854Note that I decide that the popularity count should be protected by the
855``cache_lock`` rather than the per-object lock: this is because it (like
856the :c:type:`struct list_head <list_head>` inside the object)
857is logically part of the infrastructure. This way, I don't need to grab
858the lock of every object in __cache_add() when seeking
859the least popular.
860
861I also decided that the id member is unchangeable, so I don't need to
862grab each object lock in __cache_find() to examine the
863id: the object lock is only used by a caller who wants to read or write
864the name field.
865
866Note also that I added a comment describing what data was protected by
867which locks. This is extremely important, as it describes the runtime
868behavior of the code, and can be hard to gain from just reading. And as
869Alan Cox says, “Lock data, not code”.
870
871Common Problems
872===============
873
874Deadlock: Simple and Advanced
875-----------------------------
876
877There is a coding bug where a piece of code tries to grab a spinlock
878twice: it will spin forever, waiting for the lock to be released
879(spinlocks, rwlocks and mutexes are not recursive in Linux). This is
880trivial to diagnose: not a
881stay-up-five-nights-talk-to-fluffy-code-bunnies kind of problem.
882
883For a slightly more complex case, imagine you have a region shared by a
884softirq and user context. If you use a spin_lock() call
885to protect it, it is possible that the user context will be interrupted
886by the softirq while it holds the lock, and the softirq will then spin
887forever trying to get the same lock.
888
889Both of these are called deadlock, and as shown above, it can occur even
890with a single CPU (although not on UP compiles, since spinlocks vanish
891on kernel compiles with ``CONFIG_SMP``\ =n. You'll still get data
892corruption in the second example).
893
894This complete lockup is easy to diagnose: on SMP boxes the watchdog
895timer or compiling with ``DEBUG_SPINLOCK`` set
896(``include/linux/spinlock.h``) will show this up immediately when it
897happens.
898
899A more complex problem is the so-called 'deadly embrace', involving two
900or more locks. Say you have a hash table: each entry in the table is a
901spinlock, and a chain of hashed objects. Inside a softirq handler, you
902sometimes want to alter an object from one place in the hash to another:
903you grab the spinlock of the old hash chain and the spinlock of the new
904hash chain, and delete the object from the old one, and insert it in the
905new one.
906
907There are two problems here. First, if your code ever tries to move the
908object to the same chain, it will deadlock with itself as it tries to
909lock it twice. Secondly, if the same softirq on another CPU is trying to
910move another object in the reverse direction, the following could
911happen:
912
913+-----------------------+-----------------------+
914| CPU 1 | CPU 2 |
915+=======================+=======================+
916| Grab lock A -> OK | Grab lock B -> OK |
917+-----------------------+-----------------------+
918| Grab lock B -> spin | Grab lock A -> spin |
919+-----------------------+-----------------------+
920
921Table: Consequences
922
923The two CPUs will spin forever, waiting for the other to give up their
924lock. It will look, smell, and feel like a crash.
925
926Preventing Deadlock
927-------------------
928
929Textbooks will tell you that if you always lock in the same order, you
930will never get this kind of deadlock. Practice will tell you that this
931approach doesn't scale: when I create a new lock, I don't understand
932enough of the kernel to figure out where in the 5000 lock hierarchy it
933will fit.
934
935The best locks are encapsulated: they never get exposed in headers, and
936are never held around calls to non-trivial functions outside the same
937file. You can read through this code and see that it will never
938deadlock, because it never tries to grab another lock while it has that
939one. People using your code don't even need to know you are using a
940lock.
941
942A classic problem here is when you provide callbacks or hooks: if you
943call these with the lock held, you risk simple deadlock, or a deadly
944embrace (who knows what the callback will do?).
945
946Overzealous Prevention Of Deadlocks
947~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
948
949Deadlocks are problematic, but not as bad as data corruption. Code which
950grabs a read lock, searches a list, fails to find what it wants, drops
951the read lock, grabs a write lock and inserts the object has a race
952condition.
953
954Racing Timers: A Kernel Pastime
955-------------------------------
956
957Timers can produce their own special problems with races. Consider a
958collection of objects (list, hash, etc) where each object has a timer
959which is due to destroy it.
960
961If you want to destroy the entire collection (say on module removal),
962you might do the following::
963
964 /* THIS CODE BAD BAD BAD BAD: IF IT WAS ANY WORSE IT WOULD USE
965 HUNGARIAN NOTATION */
966 spin_lock_bh(&list_lock);
967
968 while (list) {
969 struct foo *next = list->next;
970 timer_delete(&list->timer);
971 kfree(list);
972 list = next;
973 }
974
975 spin_unlock_bh(&list_lock);
976
977
978Sooner or later, this will crash on SMP, because a timer can have just
979gone off before the spin_lock_bh(), and it will only get
980the lock after we spin_unlock_bh(), and then try to free
981the element (which has already been freed!).
982
983This can be avoided by checking the result of
984timer_delete(): if it returns 1, the timer has been deleted.
985If 0, it means (in this case) that it is currently running, so we can
986do::
987
988 retry:
989 spin_lock_bh(&list_lock);
990
991 while (list) {
992 struct foo *next = list->next;
993 if (!timer_delete(&list->timer)) {
994 /* Give timer a chance to delete this */
995 spin_unlock_bh(&list_lock);
996 goto retry;
997 }
998 kfree(list);
999 list = next;
1000 }
1001
1002 spin_unlock_bh(&list_lock);
1003
1004
1005Another common problem is deleting timers which restart themselves (by
1006calling add_timer() at the end of their timer function).
1007Because this is a fairly common case which is prone to races, you should
1008use timer_delete_sync() (``include/linux/timer.h``) to handle this case.
1009
1010Before freeing a timer, timer_shutdown() or timer_shutdown_sync() should be
1011called which will keep it from being rearmed. Any subsequent attempt to
1012rearm the timer will be silently ignored by the core code.
1013
1014
1015Locking Speed
1016=============
1017
1018There are three main things to worry about when considering speed of
1019some code which does locking. First is concurrency: how many things are
1020going to be waiting while someone else is holding a lock. Second is the
1021time taken to actually acquire and release an uncontended lock. Third is
1022using fewer, or smarter locks. I'm assuming that the lock is used fairly
1023often: otherwise, you wouldn't be concerned about efficiency.
1024
1025Concurrency depends on how long the lock is usually held: you should
1026hold the lock for as long as needed, but no longer. In the cache
1027example, we always create the object without the lock held, and then
1028grab the lock only when we are ready to insert it in the list.
1029
1030Acquisition times depend on how much damage the lock operations do to
1031the pipeline (pipeline stalls) and how likely it is that this CPU was
1032the last one to grab the lock (ie. is the lock cache-hot for this CPU):
1033on a machine with more CPUs, this likelihood drops fast. Consider a
1034700MHz Intel Pentium III: an instruction takes about 0.7ns, an atomic
1035increment takes about 58ns, a lock which is cache-hot on this CPU takes
1036160ns, and a cacheline transfer from another CPU takes an additional 170
1037to 360ns. (These figures from Paul McKenney's `Linux Journal RCU
1038article <http://www.linuxjournal.com/article.php?sid=6993>`__).
1039
1040These two aims conflict: holding a lock for a short time might be done
1041by splitting locks into parts (such as in our final per-object-lock
1042example), but this increases the number of lock acquisitions, and the
1043results are often slower than having a single lock. This is another
1044reason to advocate locking simplicity.
1045
1046The third concern is addressed below: there are some methods to reduce
1047the amount of locking which needs to be done.
1048
1049Read/Write Lock Variants
1050------------------------
1051
1052Both spinlocks and mutexes have read/write variants: ``rwlock_t`` and
1053:c:type:`struct rw_semaphore <rw_semaphore>`. These divide
1054users into two classes: the readers and the writers. If you are only
1055reading the data, you can get a read lock, but to write to the data you
1056need the write lock. Many people can hold a read lock, but a writer must
1057be sole holder.
1058
1059If your code divides neatly along reader/writer lines (as our cache code
1060does), and the lock is held by readers for significant lengths of time,
1061using these locks can help. They are slightly slower than the normal
1062locks though, so in practice ``rwlock_t`` is not usually worthwhile.
1063
1064Avoiding Locks: Read Copy Update
1065--------------------------------
1066
1067There is a special method of read/write locking called Read Copy Update.
1068Using RCU, the readers can avoid taking a lock altogether: as we expect
1069our cache to be read more often than updated (otherwise the cache is a
1070waste of time), it is a candidate for this optimization.
1071
1072How do we get rid of read locks? Getting rid of read locks means that
1073writers may be changing the list underneath the readers. That is
1074actually quite simple: we can read a linked list while an element is
1075being added if the writer adds the element very carefully. For example,
1076adding ``new`` to a single linked list called ``list``::
1077
1078 new->next = list->next;
1079 wmb();
1080 list->next = new;
1081
1082
1083The wmb() is a write memory barrier. It ensures that the
1084first operation (setting the new element's ``next`` pointer) is complete
1085and will be seen by all CPUs, before the second operation is (putting
1086the new element into the list). This is important, since modern
1087compilers and modern CPUs can both reorder instructions unless told
1088otherwise: we want a reader to either not see the new element at all, or
1089see the new element with the ``next`` pointer correctly pointing at the
1090rest of the list.
1091
1092Fortunately, there is a function to do this for standard
1093:c:type:`struct list_head <list_head>` lists:
1094list_add_rcu() (``include/linux/list.h``).
1095
1096Removing an element from the list is even simpler: we replace the
1097pointer to the old element with a pointer to its successor, and readers
1098will either see it, or skip over it.
1099
1100::
1101
1102 list->next = old->next;
1103
1104
1105There is list_del_rcu() (``include/linux/list.h``) which
1106does this (the normal version poisons the old object, which we don't
1107want).
1108
1109The reader must also be careful: some CPUs can look through the ``next``
1110pointer to start reading the contents of the next element early, but
1111don't realize that the pre-fetched contents is wrong when the ``next``
1112pointer changes underneath them. Once again, there is a
1113list_for_each_entry_rcu() (``include/linux/list.h``)
1114to help you. Of course, writers can just use
1115list_for_each_entry(), since there cannot be two
1116simultaneous writers.
1117
1118Our final dilemma is this: when can we actually destroy the removed
1119element? Remember, a reader might be stepping through this element in
1120the list right now: if we free this element and the ``next`` pointer
1121changes, the reader will jump off into garbage and crash. We need to
1122wait until we know that all the readers who were traversing the list
1123when we deleted the element are finished. We use
1124call_rcu() to register a callback which will actually
1125destroy the object once all pre-existing readers are finished.
1126Alternatively, synchronize_rcu() may be used to block
1127until all pre-existing are finished.
1128
1129But how does Read Copy Update know when the readers are finished? The
1130method is this: firstly, the readers always traverse the list inside
1131rcu_read_lock()/rcu_read_unlock() pairs:
1132these simply disable preemption so the reader won't go to sleep while
1133reading the list.
1134
1135RCU then waits until every other CPU has slept at least once: since
1136readers cannot sleep, we know that any readers which were traversing the
1137list during the deletion are finished, and the callback is triggered.
1138The real Read Copy Update code is a little more optimized than this, but
1139this is the fundamental idea.
1140
1141::
1142
1143 --- cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100
1144 +++ cache.c.rcupdate 2003-12-11 17:55:14.000000000 +1100
1145 @@ -1,15 +1,18 @@
1146 #include <linux/list.h>
1147 #include <linux/slab.h>
1148 #include <linux/string.h>
1149 +#include <linux/rcupdate.h>
1150 #include <linux/mutex.h>
1151 #include <asm/errno.h>
1152
1153 struct object
1154 {
1155 - /* These two protected by cache_lock. */
1156 + /* This is protected by RCU */
1157 struct list_head list;
1158 int popularity;
1159
1160 + struct rcu_head rcu;
1161 +
1162 atomic_t refcnt;
1163
1164 /* Doesn't change once created. */
1165 @@ -40,7 +43,7 @@
1166 {
1167 struct object *i;
1168
1169 - list_for_each_entry(i, &cache, list) {
1170 + list_for_each_entry_rcu(i, &cache, list) {
1171 if (i->id == id) {
1172 i->popularity++;
1173 return i;
1174 @@ -49,19 +52,25 @@
1175 return NULL;
1176 }
1177
1178 +/* Final discard done once we know no readers are looking. */
1179 +static void cache_delete_rcu(void *arg)
1180 +{
1181 + object_put(arg);
1182 +}
1183 +
1184 /* Must be holding cache_lock */
1185 static void __cache_delete(struct object *obj)
1186 {
1187 BUG_ON(!obj);
1188 - list_del(&obj->list);
1189 - object_put(obj);
1190 + list_del_rcu(&obj->list);
1191 cache_num--;
1192 + call_rcu(&obj->rcu, cache_delete_rcu);
1193 }
1194
1195 /* Must be holding cache_lock */
1196 static void __cache_add(struct object *obj)
1197 {
1198 - list_add(&obj->list, &cache);
1199 + list_add_rcu(&obj->list, &cache);
1200 if (++cache_num > MAX_CACHE_SIZE) {
1201 struct object *i, *outcast = NULL;
1202 list_for_each_entry(i, &cache, list) {
1203 @@ -104,12 +114,11 @@
1204 struct object *cache_find(int id)
1205 {
1206 struct object *obj;
1207 - unsigned long flags;
1208
1209 - spin_lock_irqsave(&cache_lock, flags);
1210 + rcu_read_lock();
1211 obj = __cache_find(id);
1212 if (obj)
1213 object_get(obj);
1214 - spin_unlock_irqrestore(&cache_lock, flags);
1215 + rcu_read_unlock();
1216 return obj;
1217 }
1218
1219Note that the reader will alter the popularity member in
1220__cache_find(), and now it doesn't hold a lock. One
1221solution would be to make it an ``atomic_t``, but for this usage, we
1222don't really care about races: an approximate result is good enough, so
1223I didn't change it.
1224
1225The result is that cache_find() requires no
1226synchronization with any other functions, so is almost as fast on SMP as
1227it would be on UP.
1228
1229There is a further optimization possible here: remember our original
1230cache code, where there were no reference counts and the caller simply
1231held the lock whenever using the object? This is still possible: if you
1232hold the lock, no one can delete the object, so you don't need to get
1233and put the reference count.
1234
1235Now, because the 'read lock' in RCU is simply disabling preemption, a
1236caller which always has preemption disabled between calling
1237cache_find() and object_put() does not
1238need to actually get and put the reference count: we could expose
1239__cache_find() by making it non-static, and such
1240callers could simply call that.
1241
1242The benefit here is that the reference count is not written to: the
1243object is not altered in any way, which is much faster on SMP machines
1244due to caching.
1245
1246Per-CPU Data
1247------------
1248
1249Another technique for avoiding locking which is used fairly widely is to
1250duplicate information for each CPU. For example, if you wanted to keep a
1251count of a common condition, you could use a spin lock and a single
1252counter. Nice and simple.
1253
1254If that was too slow (it's usually not, but if you've got a really big
1255machine to test on and can show that it is), you could instead use a
1256counter for each CPU, then none of them need an exclusive lock. See
1257DEFINE_PER_CPU(), get_cpu_var() and
1258put_cpu_var() (``include/linux/percpu.h``).
1259
1260Of particular use for simple per-cpu counters is the ``local_t`` type,
1261and the cpu_local_inc() and related functions, which are
1262more efficient than simple code on some architectures
1263(``include/asm/local.h``).
1264
1265Note that there is no simple, reliable way of getting an exact value of
1266such a counter, without introducing more locks. This is not a problem
1267for some uses.
1268
1269Data Which Mostly Used By An IRQ Handler
1270----------------------------------------
1271
1272If data is always accessed from within the same IRQ handler, you don't
1273need a lock at all: the kernel already guarantees that the irq handler
1274will not run simultaneously on multiple CPUs.
1275
1276Manfred Spraul points out that you can still do this, even if the data
1277is very occasionally accessed in user context or softirqs/tasklets. The
1278irq handler doesn't use a lock, and all other accesses are done as so::
1279
1280 mutex_lock(&lock);
1281 disable_irq(irq);
1282 ...
1283 enable_irq(irq);
1284 mutex_unlock(&lock);
1285
1286The disable_irq() prevents the irq handler from running
1287(and waits for it to finish if it's currently running on other CPUs).
1288The spinlock prevents any other accesses happening at the same time.
1289Naturally, this is slower than just a spin_lock_irq()
1290call, so it only makes sense if this type of access happens extremely
1291rarely.
1292
1293What Functions Are Safe To Call From Interrupts?
1294================================================
1295
1296Many functions in the kernel sleep (ie. call schedule()) directly or
1297indirectly: you can never call them while holding a spinlock, or with
1298preemption disabled. This also means you need to be in user context:
1299calling them from an interrupt is illegal.
1300
1301Some Functions Which Sleep
1302--------------------------
1303
1304The most common ones are listed below, but you usually have to read the
1305code to find out if other calls are safe. If everyone else who calls it
1306can sleep, you probably need to be able to sleep, too. In particular,
1307registration and deregistration functions usually expect to be called
1308from user context, and can sleep.
1309
1310- Accesses to userspace:
1311
1312 - copy_from_user()
1313
1314 - copy_to_user()
1315
1316 - get_user()
1317
1318 - put_user()
1319
1320- kmalloc(GP_KERNEL) <kmalloc>`
1321
1322- mutex_lock_interruptible() and
1323 mutex_lock()
1324
1325 There is a mutex_trylock() which does not sleep.
1326 Still, it must not be used inside interrupt context since its
1327 implementation is not safe for that. mutex_unlock()
1328 will also never sleep. It cannot be used in interrupt context either
1329 since a mutex must be released by the same task that acquired it.
1330
1331Some Functions Which Don't Sleep
1332--------------------------------
1333
1334Some functions are safe to call from any context, or holding almost any
1335lock.
1336
1337- printk()
1338
1339- kfree()
1340
1341- add_timer() and timer_delete()
1342
1343Mutex API reference
1344===================
1345
1346.. kernel-doc:: include/linux/mutex.h
1347 :internal:
1348
1349.. kernel-doc:: kernel/locking/mutex.c
1350 :export:
1351
1352Futex API reference
1353===================
1354
1355.. kernel-doc:: kernel/futex/core.c
1356 :internal:
1357
1358.. kernel-doc:: kernel/futex/futex.h
1359 :internal:
1360
1361.. kernel-doc:: kernel/futex/pi.c
1362 :internal:
1363
1364.. kernel-doc:: kernel/futex/requeue.c
1365 :internal:
1366
1367.. kernel-doc:: kernel/futex/waitwake.c
1368 :internal:
1369
1370Further reading
1371===============
1372
1373- ``Documentation/locking/spinlocks.rst``: Linus Torvalds' spinlocking
1374 tutorial in the kernel sources.
1375
1376- Unix Systems for Modern Architectures: Symmetric Multiprocessing and
1377 Caching for Kernel Programmers:
1378
1379 Curt Schimmel's very good introduction to kernel level locking (not
1380 written for Linux, but nearly everything applies). The book is
1381 expensive, but really worth every penny to understand SMP locking.
1382 [ISBN: 0201633388]
1383
1384Thanks
1385======
1386
1387Thanks to Telsa Gwynne for DocBooking, neatening and adding style.
1388
1389Thanks to Martin Pool, Philipp Rumpf, Stephen Rothwell, Paul Mackerras,
1390Ruedi Aschwanden, Alan Cox, Manfred Spraul, Tim Waugh, Pete Zaitcev,
1391James Morris, Robert Love, Paul McKenney, John Ashby for proofreading,
1392correcting, flaming, commenting.
1393
1394Thanks to the cabal for having no influence on this document.
1395
1396Glossary
1397========
1398
1399preemption
1400 Prior to 2.5, or when ``CONFIG_PREEMPT`` is unset, processes in user
1401 context inside the kernel would not preempt each other (ie. you had that
1402 CPU until you gave it up, except for interrupts). With the addition of
1403 ``CONFIG_PREEMPT`` in 2.5.4, this changed: when in user context, higher
1404 priority tasks can "cut in": spinlocks were changed to disable
1405 preemption, even on UP.
1406
1407bh
1408 Bottom Half: for historical reasons, functions with '_bh' in them often
1409 now refer to any software interrupt, e.g. spin_lock_bh()
1410 blocks any software interrupt on the current CPU. Bottom halves are
1411 deprecated, and will eventually be replaced by tasklets. Only one bottom
1412 half will be running at any time.
1413
1414Hardware Interrupt / Hardware IRQ
1415 Hardware interrupt request. in_hardirq() returns true in a
1416 hardware interrupt handler.
1417
1418Interrupt Context
1419 Not user context: processing a hardware irq or software irq. Indicated
1420 by the in_interrupt() macro returning true.
1421
1422SMP
1423 Symmetric Multi-Processor: kernels compiled for multiple-CPU machines.
1424 (``CONFIG_SMP=y``).
1425
1426Software Interrupt / softirq
1427 Software interrupt handler. in_hardirq() returns false;
1428 in_softirq() returns true. Tasklets and softirqs both
1429 fall into the category of 'software interrupts'.
1430
1431 Strictly speaking a softirq is one of up to 32 enumerated software
1432 interrupts which can run on multiple CPUs at once. Sometimes used to
1433 refer to tasklets as well (ie. all software interrupts).
1434
1435tasklet
1436 A dynamically-registrable software interrupt, which is guaranteed to
1437 only run on one CPU at a time.
1438
1439timer
1440 A dynamically-registrable software interrupt, which is run at (or close
1441 to) a given time. When running, it is just like a tasklet (in fact, they
1442 are called from the ``TIMER_SOFTIRQ``).
1443
1444UP
1445 Uni-Processor: Non-SMP. (``CONFIG_SMP=n``).
1446
1447User Context
1448 The kernel executing on behalf of a particular process (ie. a system
1449 call or trap) or kernel thread. You can tell which process with the
1450 ``current`` macro.) Not to be confused with userspace. Can be
1451 interrupted by software or hardware interrupts.
1452
1453Userspace
1454 A process executing its own code outside the kernel.