linux-kernel - [PATCH 2/2] Update sem_otime for all operations

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Date:	Thu, 26 Sep 2013 07:08:55 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Jia He <jiakernel@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Mike Galbraith <efault@....de>,
	Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 2/2] Update sem_otime for all operations

Hi Jia,

Could you check if the patch below resolves the bug you have reported?
Just this patch, i.e. without your proposal.

I want to leave the optimization for the get_seconds() call:
We must update sem_otime in two places anyway
(either perform_atomic_semop() and exit_sem() or
do_smart_update() and semtimedop())

--
	Manfred

In commit 0a2b9d4c,the update of semaphore's sem_otime(last semop time)
was moved to one central position (do_smart_update).

But: Since do_smart_update() is only called for operations that modify
the array, this means that wait-for-zero semops do not update sem_otime
anymore.

The fix is simple:
Non-alter operations must update sem_otime.

Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
Reported-by: Jia He <jiakernel@...il.com>
---
 ipc/sem.c | 41 +++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index e5d9bb8..ec83f79 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -910,6 +910,24 @@ again:
 }
 
 /**
+ * set_semotime(sma, sops) - set sem_otime
+ * @sma: semaphore array
+ * @sops: operations that modified the array, may be NULL
+ *
+ * sem_otime is replicated to avoid cache line trashing.
+ * This function sets one instance to the current time.
+ */
+static void set_semotime(struct sem_array *sma, struct sembuf *sops)
+{
+	if (sops == NULL) {
+		sma->sem_base[0].sem_otime = get_seconds();
+	} else {
+		sma->sem_base[sops[0].sem_num].sem_otime =
+							get_seconds();
+	}
+}
+
+/**
  * do_smart_update(sma, sops, nsops, otime, pt) - optimized update_queue
  * @sma: semaphore array
  * @sops: operations that were performed
@@ -959,17 +977,10 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
 			}
 		}
 	}
-	if (otime) {
-		if (sops == NULL) {
-			sma->sem_base[0].sem_otime = get_seconds();
-		} else {
-			sma->sem_base[sops[0].sem_num].sem_otime =
-								get_seconds();
-		}
-	}
+	if (otime)
+		set_semotime(sma, sops);
 }
 
-
 /* The following counts are associated to each semaphore:
  *   semncnt        number of tasks waiting on semval being nonzero
  *   semzcnt        number of tasks waiting on semval being zero
@@ -1831,10 +1842,16 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 
 	error = perform_atomic_semop(sma, sops, nsops, un,
 					task_tgid_vnr(current));
-	if (error <= 0) {
-		if (alter && error == 0)
+	if (error == 0) {
+		/* If the operation was successful, then do
+		 * the required updates.
+		 */
+		if (alter)
 			do_smart_update(sma, sops, nsops, 1, &tasks);
-
+		else
+			set_semotime(sma, sops);
+	}
+	if (error <= 0) {
 		goto out_unlock_free;
 	}
 
-- 
1.8.3.1

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