mm/vmpressure.c: fix race in vmpressure_work_fn()

In some android devices, there will be a "divide by zero" exception.
vmpr->scanned could be zero before spin_lock(&vmpr->sr_lock).

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=88051

[akpm@linux-foundation.org: neaten]
Reported-by: ji_ang <ji_ang@163.com>
Cc: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index d4042e75f7c7..c5afd573d7da 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -165,6 +165,7 @@ static void vmpressure_work_fn(struct work_struct *work)
         unsigned long scanned;
         unsigned long reclaimed;
 
+        spin_lock(&vmpr->sr_lock);
         /*
          * Several contexts might be calling vmpressure(), so it is
          * possible that the work was rescheduled again before the old
@@ -173,11 +174,12 @@ static void vmpressure_work_fn(struct work_struct *work)
          * here. No need for any locks here since we don't care if
          * vmpr->reclaimed is in sync.
          */
-        if (!vmpr->scanned)
+        scanned = vmpr->scanned;
+        if (!scanned) {
+                spin_unlock(&vmpr->sr_lock);
                 return;
+        }
 
-        spin_lock(&vmpr->sr_lock);
-        scanned = vmpr->scanned;
         reclaimed = vmpr->reclaimed;
         vmpr->scanned = 0;
         vmpr->reclaimed = 0;