From: Manfred Spraul <manfred@colorfullife.com>
My patch that removed the spin_lock calls from the tail of sys_semtimedop
introduced a bug:
Before my patch was merged, every operation that altered an array called
update_queue. That call woke up threads that were waiting until a
semaphore value becomes 0. I've accidentially removed that call.
The attached patch fixes that by modifying update_queue: the function now
loops internally and wakes up all threads. The patch also removes
update_queue calls from the error path of sys_semtimedop: failed operations
do not modify the array, no need to rescan the list of waiting threads.
Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/include/linux/sem.h | 1 +
25-akpm/ipc/sem.c | 27 +++++++++++++++++++++------
2 files changed, 22 insertions(+), 6 deletions(-)
diff -puN include/linux/sem.h~fix-missing-wakeup-in-ipc-sem include/linux/sem.h
--- 25/include/linux/sem.h~fix-missing-wakeup-in-ipc-sem 2004-12-09 10:43:28.498055000 -0800
+++ 25-akpm/include/linux/sem.h 2004-12-09 10:43:28.503054240 -0800
@@ -109,6 +109,7 @@ struct sem_queue {
int id; /* internal sem id */
struct sembuf * sops; /* array of pending operations */
int nsops; /* number of operations */
+ int alter; /* does the operation alter the array? */
};
/* Each task has a list of undo requests. They are executed automatically
diff -puN ipc/sem.c~fix-missing-wakeup-in-ipc-sem ipc/sem.c
--- 25/ipc/sem.c~fix-missing-wakeup-in-ipc-sem 2004-12-09 10:43:28.499054848 -0800
+++ 25-akpm/ipc/sem.c 2004-12-09 10:43:28.504054088 -0800
@@ -358,8 +358,22 @@ static void update_queue (struct sem_arr
if (error <= 0) {
struct sem_queue *n;
remove_from_queue(sma,q);
- n = q->next;
q->status = IN_WAKEUP;
+ /*
+ * Continue scanning. The next operation
+ * that must be checked depends on the type of the
+ * completed operation:
+ * - if the operation modified the array, then
+ * restart from the head of the queue and
+ * check for threads that might be waiting
+ * for semaphore values to become 0.
+ * - if the operation didn't modify the array,
+ * then just continue.
+ */
+ if (q->alter)
+ n = sma->sem_pending;
+ else
+ n = q->next;
wake_up_process(q->sleeper);
/* hands-off: q will disappear immediately after
* writing q->status.
@@ -1119,8 +1133,11 @@ retry_undos:
goto out_unlock_free;
error = try_atomic_semop (sma, sops, nsops, un, current->tgid);
- if (error <= 0)
- goto update;
+ if (error <= 0) {
+ if (alter && error == 0)
+ update_queue (sma);
+ goto out_unlock_free;
+ }
/* We need to sleep on this operation, so we put the current
* task into the pending queue and go to sleep.
@@ -1132,6 +1149,7 @@ retry_undos:
queue.undo = un;
queue.pid = current->tgid;
queue.id = semid;
+ queue.alter = alter;
if (alter)
append_to_queue(sma ,&queue);
else
@@ -1183,9 +1201,6 @@ retry_undos:
remove_from_queue(sma,&queue);
goto out_unlock_free;
-update:
- if (alter)
- update_queue (sma);
out_unlock_free:
sem_unlock(sma);
out_free:
_