ipc/sem.c: optimize single sops when semval is zero

[linux-flexiantxendom0-natty.git] / ipc / sem.c

diff --git a/ipc/sem.c b/ipc/sem.c
index 7eb6f04..82518d6 100644 (file)
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -474,6 +474,17 @@ again:
                q = (struct sem_queue *)((char *)walk - offset);
                walk = walk->next;
 
+               /* If we are scanning the single sop, per-semaphore list of
+                * one semaphore and that semaphore is 0, then it is not
+                * necessary to scan the "alter" entries: simple increments
+                * that affect only one entry succeed immediately and cannot
+                * be in the  per semaphore pending queue, and decrements
+                * cannot be successful if the value is already 0.
+                */
+               if (semnum != -1 && sma->sem_base[semnum].semval == 0 &&
+                               q->alter)
+                       break;
+
                error = try_atomic_semop(sma, q->sops, q->nsops,
                                         q->undo, q->pid);