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 static DEFINE_PER_CPU(int, printk_context); 16 17 /* Can be preempted by NMI. */ 18 void __printk_safe_enter(void) 19 { 20 this_cpu_inc(printk_context); 21 } 22 23 /* Can be preempted by NMI. */ 24 void __printk_safe_exit(void) 25 { 26 this_cpu_dec(printk_context); 27 } 28 29 void __printk_deferred_enter(void) 30 { 31 cant_migrate(); 32 __printk_safe_enter(); 33 } 34 35 void __printk_deferred_exit(void) 36 { 37 cant_migrate(); 38 __printk_safe_exit(); 39 } 40 41 bool is_printk_legacy_deferred(void) 42 { 43 /* 44 * The per-CPU variable @printk_context can be read safely in any 45 * context. CPU migration is always disabled when set. 46 */ 47 return (force_legacy_kthread() || 48 this_cpu_read(printk_context) || 49 in_nmi()); 50 } 51 52 asmlinkage int vprintk(const char *fmt, va_list args) 53 { 54 #ifdef CONFIG_KGDB_KDB 55 /* Allow to pass printk() to kdb but avoid a recursion. */ 56 if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0)) 57 return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args); 58 #endif 59 60 /* 61 * Use the main logbuf even in NMI. But avoid calling console 62 * drivers that might have their own locks. 63 */ 64 if (is_printk_legacy_deferred()) 65 return vprintk_deferred(fmt, args); 66 67 /* No obstacles. */ 68 return vprintk_default(fmt, args); 69 } 70 EXPORT_SYMBOL(vprintk); 71