1// SPDX-License-Identifier: GPL-2.0-or-later
 2/*
 3 * printk_safe.c - Safe printk for printk-deadlock-prone contexts
 4 */
 5
 6#include <linux/preempt.h>
 7#include <linux/kdb.h>
 8#include <linux/smp.h>
 9#include <linux/cpumask.h>
10#include <linux/printk.h>
11#include <linux/kprobes.h>
12
13#include "internal.h"
14
15/* Context where printk messages are never suppressed */
16static atomic_t force_con;
17
18void printk_force_console_enter(void)
19{
20	atomic_inc(&force_con);
21}
22
23void printk_force_console_exit(void)
24{
25	atomic_dec(&force_con);
26}
27
28bool is_printk_force_console(void)
29{
30	return atomic_read(&force_con);
31}
32
33static DEFINE_PER_CPU(int, printk_context);
34
35/* Can be preempted by NMI. */
36void __printk_safe_enter(void)
37{
38	this_cpu_inc(printk_context);
39}
40
41/* Can be preempted by NMI. */
42void __printk_safe_exit(void)
43{
44	this_cpu_dec(printk_context);
45}
46
47void __printk_deferred_enter(void)
48{
49	cant_migrate();
50	__printk_safe_enter();
51}
52
53void __printk_deferred_exit(void)
54{
55	cant_migrate();
56	__printk_safe_exit();
57}
58
59bool is_printk_legacy_deferred(void)
60{
61	/*
62	 * The per-CPU variable @printk_context can be read safely in any
63	 * context. CPU migration is always disabled when set.
64	 *
65	 * A context holding the printk_cpu_sync must not spin waiting for
66	 * another CPU. For legacy printing, it could be the console_lock
67	 * or the port lock.
68	 */
69	return (force_legacy_kthread() ||
70		this_cpu_read(printk_context) ||
71		in_nmi() ||
72		is_printk_cpu_sync_owner());
73}
74
75asmlinkage int vprintk(const char *fmt, va_list args)
76{
77#ifdef CONFIG_KGDB_KDB
78	/* Allow to pass printk() to kdb but avoid a recursion. */
79	if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0))
80		return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
81#endif
82
83	/*
84	 * Use the main logbuf even in NMI. But avoid calling console
85	 * drivers that might have their own locks.
86	 */
87	if (is_printk_legacy_deferred())
88		return vprintk_deferred(fmt, args);
89
90	/* No obstacles. */
91	return vprintk_default(fmt, args);
92}
93EXPORT_SYMBOL(vprintk);