futex: Fix (possible) missed wakeup

mainline inclusion
from mainline-5.0
commit b061c38b
category: bugfix
bugzilla: 7208
CVE: NA

-------------------------------------------------

We must not rely on wake_q_add() to delay the wakeup; in particular
commit:

  1d0dcb3a ("futex: Implement lockless wakeups")

moved wake_q_add() before smp_store_release(&q->lock_ptr, NULL), which
could result in futex_wait() waking before observing ->lock_ptr ==
NULL and going back to sleep again.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1d0dcb3a ("futex: Implement lockless wakeups")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>