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 */
16 static atomic_t force_con;
17 
printk_force_console_enter(void)18 void printk_force_console_enter(void)
19 {
20 	atomic_inc(&force_con);
21 }
22 
printk_force_console_exit(void)23 void printk_force_console_exit(void)
24 {
25 	atomic_dec(&force_con);
26 }
27 
is_printk_force_console(void)28 bool is_printk_force_console(void)
29 {
30 	return atomic_read(&force_con);
31 }
32 
33 static DEFINE_PER_CPU(int, printk_context);
34 
35 /* Can be preempted by NMI. */
__printk_safe_enter(void)36 void __printk_safe_enter(void)
37 {
38 	this_cpu_inc(printk_context);
39 }
40 
41 /* Can be preempted by NMI. */
__printk_safe_exit(void)42 void __printk_safe_exit(void)
43 {
44 	this_cpu_dec(printk_context);
45 }
46 
__printk_deferred_enter(void)47 void __printk_deferred_enter(void)
48 {
49 	cant_migrate();
50 	__printk_safe_enter();
51 }
52 
__printk_deferred_exit(void)53 void __printk_deferred_exit(void)
54 {
55 	cant_migrate();
56 	__printk_safe_exit();
57 }
58 
is_printk_legacy_deferred(void)59 bool 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 
vprintk(const char * fmt,va_list args)75 asmlinkage 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 	return vprintk_default(fmt, args);
83 }
84 EXPORT_SYMBOL(vprintk);
85