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: Mikulas Patocka <mpatocka@redhat.com>
Cc: Alasdair G. Kergon <agk@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 17 insertions, 6 deletions

diff --git a/drivers/md/dm-stats.c b/drivers/md/dm-stats.c
index 8ae31e8d3d64..3d404c1371ed 100644
--- a/drivers/md/dm-stats.c
+++ b/drivers/md/dm-stats.c
@@ -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,