Commit bbf3f8cb authored by Mikulas Patocka's avatar Mikulas Patocka Committed by Mike Snitzer
Browse files

dm stats: fix possible counter corruption on 32-bit systems 



There was a deliberate race condition in dm_stat_for_entry() to avoid the
overhead of disabling and enabling interrupts.  The race could result in
some events not being counted on 64-bit architectures.

However, on 32-bit architectures, operations on long long variables are
not atomic, so the race condition could cause the counter to jump by 2^32.
Such jumps could be disruptive, so we need to do proper locking on 32-bit
architectures.

Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
Cc: Alasdair G. Kergon <agk@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent cc9d3c38

drivers/md/dm-stats.c

+17 −6
Original line number Diff line number Diff line
@@ -451,19 +451,26 @@ static void dm_stat_for_entry(struct dm_stat *s, size_t entry, 
	struct dm_stat_percpu *p;



	/*

	 * For strict correctness we should use local_irq_disable/enable

	 * For strict correctness we should use local_irq_save/restore

	 * instead of preempt_disable/enable.

	 *

	 * This is racy if the driver finishes bios from non-interrupt

	 * context as well as from interrupt context or from more different

	 * interrupts.

	 * preempt_disable/enable is racy if the driver finishes bios

	 * from non-interrupt context as well as from interrupt context

	 * or from more different interrupts.

	 *

	 * However, the race only results in not counting some events,

	 * so it is acceptable.

	 * On 64-bit architectures the race only results in not counting some

	 * events, so it is acceptable.  On 32-bit architectures the race could

	 * cause the counter going off by 2^32, so we need to do proper locking

	 * there.

	 *

	 * part_stat_lock()/part_stat_unlock() have this race too.

	 */

#if BITS_PER_LONG == 32

	unsigned long flags;

	local_irq_save(flags);

#else

	preempt_disable();

#endif

	p = &s->stat_percpu[smp_processor_id()][entry];



	if (!end) {
@@ -478,7 +485,11 @@ static void dm_stat_for_entry(struct dm_stat *s, size_t entry, 
		p->ticks[idx] += duration;

	}



#if BITS_PER_LONG == 32

	local_irq_restore(flags);

#else

	preempt_enable();

#endif

}



static void __dm_stat_bio(struct dm_stat *s, unsigned long bi_rw,

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