Operations that need access to the whole array must guarantee that there are
no simple operations ongoing. Right now this is achieved by
spin_unlock_wait(sem->lock) on all semaphores.
If complex_count is nonzero, then this spin_unlock_wait() is not necessary,
because it was already performed in the past by the thread that increased
complex_count and even though sem_perm.lock was dropped inbetween, no simple
operation could have started, because simple operations cannot start when
complex_count is non-zero.
What do you think?
The patch survived some testing.
Its not a bugfix - thus I don't know if it should go into linux-next first.
Signed-off-by: Manfred Spraul <manf...@colorfullife.com>
Cc: Mike Galbraith <bitbuc...@online.de>
Cc: Rik van Riel <r...@redhat.com>
Cc: Davidlohr Bueso <davidlohr.bu...@hp.com>
Cc: Andrew Morton <a...@linux-foundation.org>
---
ipc/sem.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/ipc/sem.c b/ipc/sem.c
index 4836ea7..5274ed1 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -248,12 +248,20 @@ static void merge_queues(struct sem_array *sma)
* Caller must own sem_perm.lock.
* New simple ops can start, because simple ops first check
* that sem_perm.lock is free.
+ * that a) sem_perm.lock is free and b) complex_count is 0.
*/
static void sem_wait_array(struct sem_array *sma)
{
int i;
struct sem *sem;
+ if (sma->complex_count) {
+ /* The thread that increased sma->complex_count waited on
+ * all sem->lock locks. Thus we don't need to wait again.
+ */
+ return;
+ }
+
for (i = 0; i < sma->sem_nsems; i++) {
sem = sma->sem_base + i;
spin_unlock_wait(&sem->lock);
--
1.8.3.1
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