The patch titled ipc/sem.c: sem preempt improve has been removed from the -mm tree. Its filename was ipc-semc-sem-preempt-improve.patch This patch was dropped because it was merged into mainline or a subsystem tree The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: ipc/sem.c: sem preempt improve From: Nick Piggin <npiggin@xxxxxxx> 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@xxxxxxx> Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> Cc: Pierre Peiffer <peifferp@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- ipc/sem.c | 38 +++++++++++++++++++++++--------------- 1 file changed, 23 insertions(+), 15 deletions(-) diff -puN ipc/sem.c~ipc-semc-sem-preempt-improve ipc/sem.c --- a/ipc/sem.c~ipc-semc-sem-preempt-improve +++ a/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 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 */ _ Patches currently in -mm which might be from npiggin@xxxxxxx are origin.patch linux-next.patch fs-anon_inodes-implement-dname.patch fs-remove-unneeded-dcache_unhashed-tricks.patch fs-improve-remountro-vs-buffercache-coherency.patch reiser4.patch fs-symlink-write_begin-allocation-context-fix-reiser4-fix.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html