linux-kernel - [PATCH] zram: fix possible race when checking idle

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Date:	Fri,  7 Aug 2015 17:03:29 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Minchan Kim <minchan@...nel.org>, Nitin Gupta <ngupta@...are.org>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: [PATCH] zram: fix possible race when checking idle_strm

Currently, when we enter the wait state due to lack of idle stream,
we check idle_strm list without holding the lock in expanding of
wait_event define. In this case, some one can see stale value and
process could fall into wait state without any upcoming wakeup process.
Although I didn't see any error related to this race, it should be fixed.

To fix it, we should check idle_strm with holding the lock and
this patch implements it.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
---
 drivers/block/zram/zcomp.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index 80a62e7..837e9c3 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -76,6 +76,17 @@ static struct zcomp_strm *zcomp_strm_alloc(struct zcomp *comp)
 	return zstrm;
 }
 
+static bool avail_idle_strm(struct zcomp_strm_multi *zs)
+{
+	int avail;
+
+	spin_lock(&zs->strm_lock);
+	avail = !list_empty(&zs->idle_strm);
+	spin_unlock(&zs->strm_lock);
+
+	return avail;
+}
+
 /*
  * get idle zcomp_strm or wait until other process release
  * (zcomp_strm_release()) one for us
@@ -97,7 +108,7 @@ static struct zcomp_strm *zcomp_strm_multi_find(struct zcomp *comp)
 		/* zstrm streams limit reached, wait for idle stream */
 		if (zs->avail_strm >= zs->max_strm) {
 			spin_unlock(&zs->strm_lock);
-			wait_event(zs->strm_wait, !list_empty(&zs->idle_strm));
+			wait_event(zs->strm_wait, avail_idle_strm(zs));
 			continue;
 		}
 		/* allocate new zstrm stream */
@@ -109,7 +120,7 @@ static struct zcomp_strm *zcomp_strm_multi_find(struct zcomp *comp)
 			spin_lock(&zs->strm_lock);
 			zs->avail_strm--;
 			spin_unlock(&zs->strm_lock);
-			wait_event(zs->strm_wait, !list_empty(&zs->idle_strm));
+			wait_event(zs->strm_wait, avail_idle_strm(zs));
 			continue;
 		}
 		break;
-- 
1.9.1

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