linux-kernel - [tip:core/urgent] futex: Fix spurious wakeup for requeue

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Message-ID: <tip-11df6dddcbc38affb7473aad3d962baf8414a947@git.kernel.org>
Date:	Wed, 28 Oct 2009 19:36:38 GMT
From:	tip-bot for Thomas Gleixner <tglx@...utronix.de>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, dvhltc@...ibm.com, hpa@...or.com,
	mingo@...hat.com, johnstul@...ux.vnet.ibm.com,
	eric.dumazet@...il.com, peterz@...radead.org, dino@...ibm.com,
	tglx@...utronix.de
Subject: [tip:core/urgent] futex: Fix spurious wakeup for requeue_pi really

Commit-ID:  11df6dddcbc38affb7473aad3d962baf8414a947
Gitweb:     http://git.kernel.org/tip/11df6dddcbc38affb7473aad3d962baf8414a947
Author:     Thomas Gleixner <tglx@...utronix.de>
AuthorDate: Wed, 28 Oct 2009 20:26:48 +0100
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Wed, 28 Oct 2009 20:34:34 +0100

futex: Fix spurious wakeup for requeue_pi really

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@...ibm.com>

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@...ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: John Stultz <johnstul@...ux.vnet.ibm.com>
Cc: Dinakar Guniguntala <dino@...ibm.com>
LKML-Reference: <4AE23C74.1090502@...ibm.com>
Cc: stable@...nel.org
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

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

diff --git a/kernel/futex.c b/kernel/futex.c
index 642f3bb..fb65e82 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2127,7 +2127,7 @@ int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
 		plist_del(&q->list, &q->list.plist);
 
 		/* Handle spurious wakeups gracefully */
-		ret = -EAGAIN;
+		ret = -EWOULDBLOCK;
 		if (timeout && !timeout->task)
 			ret = -ETIMEDOUT;
 		else if (signal_pending(current))
@@ -2208,7 +2208,6 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
 	debug_rt_mutex_init_waiter(&rt_waiter);
 	rt_waiter.task = NULL;
 
-retry:
 	key2 = FUTEX_KEY_INIT;
 	ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
 	if (unlikely(ret != 0))
@@ -2303,9 +2302,6 @@ out_put_keys:
 out_key2:
 	put_futex_key(fshared, &key2);
 
-	/* Spurious wakeup ? */
-	if (ret == -EAGAIN)
-		goto retry;
 out:
 	if (to) {
 		hrtimer_cancel(&to->timer);
--
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