From 8fe8f545c6d753ead15e1f4919d39e8f9bb49629 Mon Sep 17 00:00:00 2001 From: Michel Lespinasse Date: Sun, 6 Mar 2011 18:07:50 -0800 Subject: futex: Update futex_wait_setup comments about locking Reviving a cleanup I had done about a year ago as part of a larger futex_set_wait proposal. Over the years, the locking of the hashed futex queue got improved, so that some of the "rare but normal" race conditions described in comments can't actually happen anymore. Signed-off-by: Michel Lespinasse Cc: Linus Torvalds Cc: Darren Hart Cc: Peter Zijlstra LKML-Reference: <20110307020750.GA31188@google.com> Signed-off-by: Thomas Gleixner --- kernel/futex.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index b766d28accd6..3184d3b9cadf 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1781,13 +1781,14 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags, * * The basic logical guarantee of a futex is that it blocks ONLY * if cond(var) is known to be true at the time of blocking, for - * any cond. If we queued after testing *uaddr, that would open - * a race condition where we could block indefinitely with + * any cond. If we locked the hash-bucket after testing *uaddr, that + * would open a race condition where we could block indefinitely with * cond(var) false, which would violate the guarantee. * - * A consequence is that futex_wait() can return zero and absorb - * a wakeup when *uaddr != val on entry to the syscall. This is - * rare, but normal. + * On the other hand, we insert q and release the hash-bucket only + * after testing *uaddr. This guarantees that futex_wait() will NOT + * absorb a wakeup if *uaddr does not match the desired values + * while the syscall executes. */ retry: ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key); -- cgit v1.2.3