[PATCH] shm: optimize exit_shm()

[Vasiliy Kulikov]

We may check .in_use == 0 without holding the rw_mutex as .in_use is int
and reads of ints are atomic.  As .in_use may be changed to zero while current
process was sleeping in down_write(), we should check .in_use once again after
down_write().

Signed-off-by: Vasiliy Kulikov <segoon@xxxxxxxxxxxx>
---
 ipc/shm.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index 4e3c883..855ddc0 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -304,6 +304,9 @@ void exit_shm(struct task_struct *task)
 {
 	struct ipc_namespace *ns = task->nsproxy->ipc_ns;
 
+	if (shm_ids(ns).in_use == 0)
+		return;
+
 	/* Destroy all already created segments, but not mapped yet */
 	down_write(&shm_ids(ns).rw_mutex);
 	if (shm_ids(ns).in_use)
-- 
1.7.0.4

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