futex: Fix spurious wakeup for requeue_pi really (11df6ddd) · Commits · 戴 / test

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

+1 −5

Original line number	Original line	Diff line number	Diff line
	@@ -2127,7 +2127,7 @@ int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
	plist_del(&q->list, &q->list.plist);		plist_del(&q->list, &q->list.plist);

	/* Handle spurious wakeups gracefully */		/* Handle spurious wakeups gracefully */
	ret = -EAGAIN;		ret = -EWOULDBLOCK;
	if (timeout && !timeout->task)		if (timeout && !timeout->task)
	ret = -ETIMEDOUT;		ret = -ETIMEDOUT;
	else if (signal_pending(current))		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);		debug_rt_mutex_init_waiter(&rt_waiter);
	rt_waiter.task = NULL;		rt_waiter.task = NULL;

	retry:
	key2 = FUTEX_KEY_INIT;		key2 = FUTEX_KEY_INIT;
	ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);		ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
	if (unlikely(ret != 0))		if (unlikely(ret != 0))
	@@ -2303,9 +2302,6 @@ out_put_keys:
	out_key2:		out_key2:
	put_futex_key(fshared, &key2);		put_futex_key(fshared, &key2);

	/* Spurious wakeup ? */
	if (ret == -EAGAIN)
	goto retry;
	out:		out:
	if (to) {		if (to) {
	hrtimer_cancel(&to->timer);		hrtimer_cancel(&to->timer);

Admin message