Commit fb842b00 authored by Huang Ying's avatar Huang Ying Committed by Linus Torvalds
Browse files

printk: use RCU to prevent potential lock contention in kmsg_dump

dump_list_lock is used to protect dump_list in kmsg_dumper implementation,
kmsg_dump() uses it to traverse dump_list too.  But if there is contention
on the lock, kmsg_dump() will fail, and the valuable kernel message may be

This patch solves this issue with RCU.  Because kmsg_dump() only read the
list, no lock is needed in kmsg_dump().  So that kmsg_dump() will never
fail because of lock contention.
Signed-off-by: default avatarHuang Ying <>
Cc: "Paul E. McKenney" <>
Cc: Ingo Molnar <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent f0f2c2b5
......@@ -39,6 +39,7 @@
#include <linux/syslog.h>
#include <linux/cpu.h>
#include <linux/notifier.h>
#include <linux/rculist.h>
#include <asm/uaccess.h>
......@@ -1502,7 +1503,7 @@ int kmsg_dump_register(struct kmsg_dumper *dumper)
/* Don't allow registering multiple times */
if (!dumper->registered) {
dumper->registered = 1;
list_add_tail(&dumper->list, &dump_list);
list_add_tail_rcu(&dumper->list, &dump_list);
err = 0;
spin_unlock_irqrestore(&dump_list_lock, flags);
......@@ -1526,33 +1527,16 @@ int kmsg_dump_unregister(struct kmsg_dumper *dumper)
spin_lock_irqsave(&dump_list_lock, flags);
if (dumper->registered) {
dumper->registered = 0;
err = 0;
spin_unlock_irqrestore(&dump_list_lock, flags);
return err;
static const char * const kmsg_reasons[] = {
[KMSG_DUMP_OOPS] = "oops",
[KMSG_DUMP_PANIC] = "panic",
[KMSG_DUMP_KEXEC] = "kexec",
[KMSG_DUMP_RESTART] = "restart",
[KMSG_DUMP_HALT] = "halt",
[KMSG_DUMP_POWEROFF] = "poweroff",
[KMSG_DUMP_EMERG] = "emergency_restart",
static const char *kmsg_to_str(enum kmsg_dump_reason reason)
if (reason >= ARRAY_SIZE(kmsg_reasons) || reason < 0)
return "unknown";
return kmsg_reasons[reason];
* kmsg_dump - dump kernel log to kernel message dumpers.
* @reason: the reason (oops, panic etc) for dumping
......@@ -1591,13 +1575,9 @@ void kmsg_dump(enum kmsg_dump_reason reason)
l2 = chars;
if (!spin_trylock_irqsave(&dump_list_lock, flags)) {
printk(KERN_ERR "dump_kmsg: dump list lock is held during %s, skipping dump\n",
list_for_each_entry(dumper, &dump_list, list)
list_for_each_entry_rcu(dumper, &dump_list, list)
dumper->dump(dumper, reason, s1, l1, s2, l2);
spin_unlock_irqrestore(&dump_list_lock, flags);
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