[patch 3/4] ipc: sem preempt improve

npiggin
2009-08-11T11:57:06Z

[-- Attachment #1: ipc-sem-preempt-improve.patch --]
[-- Type: text/plain, Size: 2573 bytes --]

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>
---
ipc/sem.c | 38 +++++++++++++++++++++++---------------
1 file changed, 23 insertions(+), 15 deletions(-)

Index: linux-2.6/ipc/sem.c
===================================================================
--- linux-2.6.orig/ipc/sem.c
+++ linux-2.6/ipc/sem.c
@@ -397,6 +397,27 @@ 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.
*/
@@ -428,17 +449,7 @@ 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;
}
@@ -522,10 +533,7 @@ static void freeary(struct ipc_namespace
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 */