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@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>

1 files changed, 1 insertions, 5 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 642f3bbaacc7..fb65e822fc41 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);