[12/99] futex: Fix spurious wakeup for requeue_pi really

From: Greg KH
Date: Fri Nov 06 2009 - 17:47:28 EST

2.6.31-stable review patch. If anyone has any objections, please let us know.

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit 11df6dddcbc38affb7473aad3d962baf8414a947 upstream.

The requeue_pi path doesn't use unqueue_me() (and the racy lock_ptr ==
NULL test) nor does it use the wake_list of futex_wake() which where
the reason for commit 41890f2 (futex: Handle spurious wake up)

See debugging discussing on LKML Message-ID: <4AD4080C.20703@xxxxxxxxxx>

The changes in this fix to the wait_requeue_pi path were considered to
be a likely unecessary, but harmless safety net. But it turns out that
due to the fact that for unknown $@#!*( reasons EWOULDBLOCK is defined
as EAGAIN we built an endless loop in the code path which returns
correctly EWOULDBLOCK.

Spurious wakeups in wait_requeue_pi code path are unlikely so we do
the easy solution and return EWOULDBLOCK^WEAGAIN to user space and let
it deal with the spurious wakeup.

Cc: Darren Hart <dvhltc@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Cc: John Stultz <johnstul@xxxxxxxxxxxxxxxxxx>
Cc: Dinakar Guniguntala <dino@xxxxxxxxxx>
LKML-Reference: <4AE23C74.1090502@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

kernel/futex.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2103,7 +2103,7 @@ int handle_early_requeue_pi_wakeup(struc
plist_del(&q->list, &q->list.plist);

/* Handle spurious wakeups gracefully */
- ret = -EAGAIN;
if (timeout && !timeout->task)
else if (signal_pending(current))
@@ -2184,7 +2184,6 @@ static int futex_wait_requeue_pi(u32 __u
rt_waiter.task = NULL;

ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
if (unlikely(ret != 0))
@@ -2282,9 +2281,6 @@ out_put_keys:
put_futex_key(fshared, &key2);

- /* Spurious wakeup ? */
- if (ret == -EAGAIN)
- goto retry;
if (to) {

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