Re: 2.6.32-rc5-mmotm1101 - unkillable processes stuck in futex.

From: Thomas Gleixner
Date: Thu Nov 05 2009 - 14:20:51 EST

On Thu, 5 Nov 2009, Valdis.Kletnieks@xxxxxx wrote:

> (Hmm.. I seem to be on a roll on this -mmotm, breaking all sorts of stuff.. :)
> Am cc'ing Thomas and Darren because their names were attached to commits in
> the origin.patch that touched futex.c

Looks like you are hitting the bug we fixed last week.


commit 11df6dddcbc38affb7473aad3d962baf8414a947
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date: Wed Oct 28 20:26:48 2009 +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@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>
Cc: stable@xxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

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;
if (timeout && !timeout->task)
else if (signal_pending(current))
@@ -2208,7 +2208,6 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
rt_waiter.task = NULL;

ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
if (unlikely(ret != 0))
@@ -2303,9 +2302,6 @@ out_put_keys:
put_futex_key(fshared, &key2);

- /* Spurious wakeup ? */
- if (ret == -EAGAIN)
- goto retry;
if (to) {
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at