linux-kernel - [PATCH 1/4] ipc/sem.c: Fix missing wakeups in do_smart_update

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Date:	Sun, 26 May 2013 11:08:52 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>, hhuang@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 1/4] ipc/sem.c: Fix missing wakeups in do_smart_update_queue()

do_smart_update_queue() is called when an operation
(semop, semctl(SETVAL), semctl(SETALL), ...) modified the array.
It must check which of the sleeping tasks can proceed.

do_smart_update_queue() missed a few wakeups:
- if a sleeping complex op was completed, then all per-semaphore queues
  must be scanned - not only those that were modified by *sops
- if a sleeping simple op proceeded, then the global queue
  must be scanned again

And:
- the test for "|sops == NULL) before scanning the global
  queue is not required: If the global queue is empty, then
  it doesn't need to be scanned - regardless of the reason
  for calling do_smart_update_queue()

The patch is not optimized, i.e. even completing a wait-for-zero
operation causes a rescan. This is done to keep the patch as simple as
possible.
Avoiding unnecessary scans is implemented in the following patches.

Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
---
 ipc/sem.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index a7e40ed..70480a3 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -752,19 +752,29 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
 			int otime, struct list_head *pt)
 {
 	int i;
+	int progress;
 
-	if (sma->complex_count || sops == NULL) {
-		if (update_queue(sma, -1, pt))
+	progress = 1;
+retry_global:
+	if (sma->complex_count) {
+		if (update_queue(sma, -1, pt)) {
+			progress = 1;
 			otime = 1;
+			sops = NULL;
+		}
 	}
+	if (!progress)
+		goto done;
 
 	if (!sops) {
 		/* No semops; something special is going on. */
 		for (i = 0; i < sma->sem_nsems; i++) {
-			if (update_queue(sma, i, pt))
+			if (update_queue(sma, i, pt)) {
 				otime = 1;
+				progress = 1;
+			}
 		}
-		goto done;
+		goto done_checkretry;
 	}
 
 	/* Check the semaphores that were modified. */
@@ -772,8 +782,15 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
 		if (sops[i].sem_op > 0 ||
 			(sops[i].sem_op < 0 &&
 				sma->sem_base[sops[i].sem_num].semval == 0))
-			if (update_queue(sma, sops[i].sem_num, pt))
+			if (update_queue(sma, sops[i].sem_num, pt)) {
 				otime = 1;
+				progress = 1;
+			}
+	}
+done_checkretry:
+	if (progress) {
+		progress = 0;
+		goto retry_global;
 	}
 done:
 	if (otime)
-- 
1.8.1.4

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