ipc/sem.c: sem preempt improve
authorNick Piggin <npiggin@suse.de>
Wed, 16 Dec 2009 00:47:30 +0000 (16:47 -0800)
committerLinus Torvalds <torvalds@linux-foundation.org>
Wed, 16 Dec 2009 15:20:09 +0000 (07:20 -0800)
commitd4212093dca95c1f52197017d969cc66d5d962aa
treef52484ae3c7b3034715b5745ae2b7db3906ab022
parent9cad200c7686708b326520a45dd680a4147568a6
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>
ipc/sem.c