Re: [patch V3 56/64] futex: Correct the number of requeued waiters for PI

Thomas Gleixner
2021-08-09T08:18:26Z

On Sun, Aug 08 2021 at 10:05, Davidlohr Bueso wrote:
> On Thu, 05 Aug 2021, Thomas Gleixner wrote:
>
>>From: Thomas Gleixner <tglx@linutronix.de>
>>
>>The accounting is wrong when either the PI sanity check or the
>>requeue PI operation fails. Adjust it in the failure path.
>
> Ok fortunately these accounting errors are benign considering they
> are in error paths. This also made me wonder about the requeue PI
> top-waiter wakeup from futex_proxy_trylock_atomic(), which is always
> required with nr_wakers == 1. We account for it on the successful
> case we acquired the lock on it's behalf (and thus requeue_pi_wake_futex
> was called), but if the corresponding lookup_pi_state fails, we'll retry.
> So, shouldn't the task_count++ only be considered when we know the
> requeueing is next (a successful top_waiter acquiring the lock+pi state)?
>
> @@ -2260,7 +2260,6 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
> */
> if (ret > 0) {
> WARN_ON(pi_state);
> - task_count++;
> /*
> * If we acquired the lock, then the user space value
> * of uaddr2 should be vpid. It cannot be changed by
> @@ -2275,6 +2274,8 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
> */
> ret = lookup_pi_state(uaddr2, ret, hb2, &key2,
> &pi_state, &exiting);
> + if (!ret)
> + task_count++;
> }

Yes, but if futex_proxy_trylock_atomic() succeeds and lookup_pi_state()
fails then the user space futex value is already the VPID of the top
waiter and a retry will then fail the futex_proxy_trylock_atomic().

> switch (ret) {
>
> Also, looking at the code, I think we can use the goto retry_private
> optimization for private futexes upon futex_proxy_trylock_atomic
> lookup_pi_state errors:
>
> @@ -2290,8 +2290,11 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
> double_unlock_hb(hb1, hb2);
> hb_waiters_dec(hb2);
> ret = fault_in_user_writeable(uaddr2);
> - if (!ret)
> + if (!ret) {
> + if (!(flags & FLAGS_SHARED))
> + goto retry_private;
> goto retry;
> + }

Yes, we can, but let me stare at that code some more.

Thanks,

tglx