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>

diff --git a/drivers/md/dm-stats.c b/drivers/md/dm-stats.c
index 8ae31e8d3d64964652fd1bd8d31d35d3ed49e879..3d404c1371ed2d7e6f4fa052fd4379fbc89ec388 100644 (file)
--- 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,