kernel/printk/printk_safe.c

   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
  18 void printk_force_console_enter(void)
  19 {
  20         atomic_inc(&force_con);
  21 }
  22
  23 void printk_force_console_exit(void)
  24 {
  25         atomic_dec(&force_con);
  26 }
  27
  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. */
  36 void __printk_safe_enter(void)
  37 {
  38         this_cpu_inc(printk_context);
  39 }
  40
  41 /* Can be preempted by NMI. */
  42 void __printk_safe_exit(void)
  43 {
  44         this_cpu_dec(printk_context);
  45 }
  46
  47 void __printk_deferred_enter(void)
  48 {
  49         cant_migrate();
  50         __printk_safe_enter();
  51 }
  52
  53 void __printk_deferred_exit(void)
  54 {
  55         cant_migrate();
  56         __printk_safe_exit();
  57 }
  58
  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         return (force_legacy_kthread() ||
  66                 this_cpu_read(printk_context) ||
  67                 in_nmi());
  68 }
  69
  70 asmlinkage int vprintk(const char *fmt, va_list args)
  71 {
  72 #ifdef CONFIG_KGDB_KDB
  73         /* Allow to pass printk() to kdb but avoid a recursion. */
  74         if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0))
  75                 return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
  76 #endif
  77
  78         /*
  79          * Use the main logbuf even in NMI. But avoid calling console
  80          * drivers that might have their own locks.
  81          */
  82         if (is_printk_legacy_deferred())
  83                 return vprintk_deferred(fmt, args);
  84
  85         /* No obstacles. */
  86         return vprintk_default(fmt, args);
  87 }
  88 EXPORT_SYMBOL(vprintk);