Re: [PATCH] futex: Resend potentially swallowed owner death notification

[Peter Zijlstra]

On Sat, Nov 12, 2022 at 12:54:39AM +0300, Alexey Izbyshev wrote:
> 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 remained 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@xxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>