ipc/sem.c: sem preempt improve

The strange sysv semaphore wakeup scheme has a kind of busy-wait lock
involved, which could deadlock if preemption is enabled during the "lock".

It is an implementation detail (due to a spinlock being held) that this is
actually the case.  However if "spinlocks" are made preemptible, or if the
sem lock is changed to a sleeping lock for example, then the wakeup would
become buggy.  So this might be a bugfix for -rt kernels.

Imagine waker being preempted by wakee and never clearing IN_WAKEUP -- if
wakee has higher RT priority then there is a priority inversion deadlock.
Even if there is not a priority inversion to cause a deadlock, then there
is still time wasted spinning.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Pierre Peiffer <peifferp@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 23 insertions, 15 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index d377b3adfc3d..2705fbbc4374 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -398,6 +398,27 @@ undo:
         return result;
 }
 
+/*
+ * Wake up a process waiting on the sem queue with a given error.
+ * The queue is invalid (may not be accessed) after the function returns.
+ */
+static void wake_up_sem_queue(struct sem_queue *q, int error)
+{
+        /*
+         * Hold preempt off so that we don't get preempted and have the
+         * wakee busy-wait until we're scheduled back on. We're holding
+         * locks here so it may not strictly be needed, however if the
+         * locks become preemptible then this prevents such a problem.
+         */
+        preempt_disable();
+        q->status = IN_WAKEUP;
+        wake_up_process(q->sleeper);
+        /* hands-off: q can disappear immediately after writing q->status. */
+        smp_wmb();
+        q->status = error;
+        preempt_enable();
+}
+
 /* Go through the pending queue for the indicated semaphore
  * looking for tasks that can be completed.
  */
@@ -429,17 +450,7 @@ again:
                  *   continue.
                  */
                 alter = q->alter;
-
-                /* wake up the waiting thread */
-                q->status = IN_WAKEUP;
-
-                wake_up_process(q->sleeper);
-                /* hands-off: q will disappear immediately after
-                 * writing q->status.
-                 */
-                smp_wmb();
-                q->status = error;
-
+                wake_up_sem_queue(q, error);
                 if (alter)
                         goto again;
         }
@@ -523,10 +534,7 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
         list_for_each_entry_safe(q, tq, &sma->sem_pending, list) {
                 list_del(&q->list);
 
-                q->status = IN_WAKEUP;
-                wake_up_process(q->sleeper); /* doesn't sleep */
-                smp_wmb();
-                q->status = -EIDRM;     /* hands-off q */
+                wake_up_sem_queue(q, -EIDRM);
         }
 
         /* Remove the semaphore set from the IDR */