Commit 3342aa8e authored by John Ogness's avatar John Ogness Committed by Petr Mladek
Browse files

printk: fix cpu lock ordering 



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: default avatarJohn Ogness <john.ogness@linutronix.de>
Reviewed-by: default avatarSergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210617095051.4808-3-john.ogness@linutronix.de
parent 766c268b

kernel/printk/printk.c

+50 −3
Original line number Diff line number Diff line
@@ -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 */