futex: Resend potentially swallowed owner death notification

futex: Resend potentially swallowed owner death notification

[ Upstream commit 90d758896787048fa3d4209309d4800f3920e66f ]

Commit ca16d5bee598 ("futex: Prevent robust futex exit race") addressed
two cases when tasks waiting on a robust non-PI futex remained blocked
despite the futex not being owned anymore:

* if the owner died after writing zero to the futex word, but before
  waking up a waiter

* if a task waiting on the futex was woken up, but died before updating
  the futex word (effectively swallowing the notification without acting
  on it)

In the second case, the task could be woken up either by the previous
owner (after the futex word was reset to zero) or by the kernel (after
the OWNER_DIED bit was set and the TID part of the futex word was reset
to zero) if the previous owner died without the resetting the futex.

Because the referenced commit wakes up a potential waiter only if the
whole futex word is zero, the latter subcase remains unaddressed.

Fix this by looking only at the TID part of the futex when deciding
whether a wake up is needed.

Fixes: ca16d5bee598 ("futex: Prevent robust futex exit race")
Signed-off-by: Alexey Izbyshev <izbyshev@ispras.ru>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20221111215439.248185-1-izbyshev@ispras.ru
Signed-off-by: Sasha Levin <sashal@kernel.org>