futex: Fix spurious wakeup for requeue_pi really
Thomas Gleixner [Wed, 28 Oct 2009 19:26:48 +0000 (20:26 +0100)]
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@us.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@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: John Stultz <johnstul@linux.vnet.ibm.com>
Cc: Dinakar Guniguntala <dino@in.ibm.com>
LKML-Reference: <4AE23C74.1090502@us.ibm.com>
Cc: stable@kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/futex.c

index 642f3bb..fb65e82 100644 (file)
@@ -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);