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printk: use RCU to prevent potential lock contention in kmsg_dump

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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
lost.

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: Huang Ying <ying.huang@intel.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Huang Ying 2011-01-12 16:59:43 -08:00 committed by Linus Torvalds
parent f0f2c2b5b4
commit fb842b00c5
1 changed files with 7 additions and 27 deletions
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34
kernel/printk.c
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@ -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;
list_del(&dumper->list);
list_del_rcu(&dumper->list);
err = 0;
}
spin_unlock_irqrestore(&dump_list_lock, flags);
synchronize_rcu();
return err;
}
EXPORT_SYMBOL_GPL(kmsg_dump_unregister);
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",
kmsg_to_str(reason));
return;
}
list_for_each_entry(dumper, &dump_list, list)
rcu_read_lock();
list_for_each_entry_rcu(dumper, &dump_list, list)
dumper->dump(dumper, reason, s1, l1, s2, l2);
spin_unlock_irqrestore(&dump_list_lock, flags);
rcu_read_unlock();
}
#endif