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printk: fix cpu lock ordering

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The cpu lock implementation uses a full memory barrier to take
the lock, but no memory barriers when releasing the lock. This
means that changes performed by a lock owner may not be seen by
the next lock owner. This may have been "good enough" for use
by dump_stack() as a serialization mechanism, but it is not
enough to provide proper protection for a critical section.

Correct this problem by using acquire/release memory barriers
for lock/unlock, respectively.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210617095051.4808-3-john.ogness@linutronix.de
This commit is contained in:
John Ogness 2021-06-17 11:56:51 +02:06 committed by Petr Mladek
parent 766c268bc6
commit 3342aa8e6b
1 changed files with 50 additions and 3 deletions
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53
kernel/printk/printk.c
View File

@ -3568,10 +3568,33 @@ int __printk_cpu_trylock(void)
cpu = smp_processor_id();
old = atomic_cmpxchg(&printk_cpulock_owner, -1, cpu);
/*
* Guarantee loads and stores from this CPU when it is the lock owner
* are _not_ visible to the previous lock owner. This pairs with
* __printk_cpu_unlock:B.
*
* Memory barrier involvement:
*
* If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B, then
* __printk_cpu_unlock:A can never read from __printk_cpu_trylock:B.
*
* Relies on:
*
* RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B
* of the previous CPU
* matching
* ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B
* of this CPU
*/
old = atomic_cmpxchg_acquire(&printk_cpulock_owner, -1,
cpu); /* LMM(__printk_cpu_trylock:A) */
if (old == -1) {
/* This CPU is now the owner. */
/*
* This CPU is now the owner and begins loading/storing
* data: LMM(__printk_cpu_trylock:B)
*/
return 1;
} else if (old == cpu) {
/* This CPU is already the owner. */
atomic_inc(&printk_cpulock_nested);
@ -3596,7 +3619,31 @@ void __printk_cpu_unlock(void)
return;
}
atomic_set(&printk_cpulock_owner, -1);
/*
* This CPU is finished loading/storing data:
* LMM(__printk_cpu_unlock:A)
*/
/*
* Guarantee loads and stores from this CPU when it was the
* lock owner are visible to the next lock owner. This pairs
* with __printk_cpu_trylock:A.
*
* Memory barrier involvement:
*
* If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B,
* then __printk_cpu_trylock:B reads from __printk_cpu_unlock:A.
*
* Relies on:
*
* RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B
* of this CPU
* matching
* ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B
* of the next CPU
*/
atomic_set_release(&printk_cpulock_owner,
-1); /* LMM(__printk_cpu_unlock:B) */
}
EXPORT_SYMBOL(__printk_cpu_unlock);
#endif /* CONFIG_SMP */