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Message-ID: <20230912105745.GB13926@noisy.programming.kicks-ass.net>
Date: Tue, 12 Sep 2023 12:57:45 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de,
boqun.feng@...il.com, bristot@...hat.com, bsegall@...gle.com,
dietmar.eggemann@....com, jstultz@...gle.com,
juri.lelli@...hat.com, longman@...hat.com, mgorman@...e.de,
mingo@...hat.com, rostedt@...dmis.org, swood@...hat.com,
vincent.guittot@...aro.org, vschneid@...hat.com, will@...nel.org
Subject: Re: [PATCH v3 7/7] locking/rtmutex: Acquire the hb lock via trylock
after wait-proxylock.
On Mon, Sep 11, 2023 at 04:11:35PM +0200, Peter Zijlstra wrote:
> Now.. the reason we need hb->lock at this point is to avoid the
> wake_futex_pi() -EAGAIN case.
>
> This happens when futex_top_waiter() and rt_mutex_top_waiter() state
> becomes inconsistent. The current rules are such that this inconsistency
> will not be observed.
>
> Notably the case that needs to be avoided is where futex_lock_pi() and
> futex_unlock_pi() interleave such that unlock will fail to observe a new
> waiter.
>
> *However* the case at hand is where a waiter is leaving, in this case
> what happens is that we'll fail to observe a waiter that is already
> gone, which is harmless afaict.
>
> Would not something like the below work instead?
I forgot to say this is complete untested and hasn't evne seen a
compiler up close...
> ---
> diff --git a/kernel/futex/pi.c b/kernel/futex/pi.c
> index ce2889f12375..8c76a52da9bd 100644
> --- a/kernel/futex/pi.c
> +++ b/kernel/futex/pi.c
> @@ -610,29 +610,16 @@ int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
> /*
> * Caller must hold a reference on @pi_state.
> */
> -static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state)
> +static int wake_futex_pi(u32 __user *uaddr, u32 uval,
> + struct futex_pi_state *pi_state,
> + rt_mutex_waiter *top_waiter)
> {
> - struct rt_mutex_waiter *top_waiter;
> struct task_struct *new_owner;
> bool postunlock = false;
> DEFINE_RT_WAKE_Q(wqh);
> u32 curval, newval;
> int ret = 0;
>
> - top_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);
> - if (WARN_ON_ONCE(!top_waiter)) {
> - /*
> - * As per the comment in futex_unlock_pi() this should not happen.
> - *
> - * When this happens, give up our locks and try again, giving
> - * the futex_lock_pi() instance time to complete, either by
> - * waiting on the rtmutex or removing itself from the futex
> - * queue.
> - */
> - ret = -EAGAIN;
> - goto out_unlock;
> - }
> -
> new_owner = top_waiter->task;
>
> /*
> @@ -1039,19 +1026,27 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
> ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);
>
> cleanup:
> - spin_lock(q.lock_ptr);
> /*
> * If we failed to acquire the lock (deadlock/signal/timeout), we must
> * first acquire the hb->lock before removing the lock from the
> * rt_mutex waitqueue, such that we can keep the hb and rt_mutex wait
> * lists consistent.
> *
> - * In particular; it is important that futex_unlock_pi() can not
> - * observe this inconsistency.
> + * Cannot hold hb->lock because rt_mutex already has a waiter enqueued
> + * and hb->lock can itself try and enqueue an rt_waiter through rtlock.
> + *
> + * Doing the cleanup without holding hb->lock can cause inconsistent
> + * state between hb and pi_state, but only in the direction of seeing a
> + * waiter that is leaving.
> */
> if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter))
> ret = 0;
>
> + /*
> + * Now that the rt_waiter has been dequeued, it is safe to use
> + * spinlock/rtlock, which will enqueue a new rt_waiter.
> + */
> + spin_lock(q.lock_ptr);
> no_block:
> /*
> * Fixup the pi_state owner and possibly acquire the lock if we
> @@ -1132,6 +1127,7 @@ int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
> top_waiter = futex_top_waiter(hb, &key);
> if (top_waiter) {
> struct futex_pi_state *pi_state = top_waiter->pi_state;
> + struct rt_mutex_waiter *rt_waiter;
>
> ret = -EINVAL;
> if (!pi_state)
> @@ -1147,19 +1143,34 @@ int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
> get_pi_state(pi_state);
> /*
> * By taking wait_lock while still holding hb->lock, we ensure
> - * there is no point where we hold neither; and therefore
> - * wake_futex_p() must observe a state consistent with what we
> - * observed.
> + * there is no point where we hold neither; and thereby
> + * wake_futex_pi() must observe any new waiters.
> + *
> + * Since the cleanup: case in futex_lock_pi() removes the
> + * rt_waiter without holding hb->lock, it is possible for
> + * wake_futex_pi() to not find a waiter while the above does,
> + * in this case the waiter is on the way out and it can be
> + * ignored.
> *
> * In particular; this forces __rt_mutex_start_proxy() to
> * complete such that we're guaranteed to observe the
> - * rt_waiter. Also see the WARN in wake_futex_pi().
> + * rt_waiter.
> */
> raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
> +
> + /*
> + * Futex vs rt_mutex waiter state -- if there are on rt_mutex
> + * waiters even though futex thinkgs there are, then the waiter
> + * is leaving and the uncontended path is safe to take.
> + */
> + rt_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);
> + if (!rt_waiter)
> + goto do_uncontended;
That ^ needs to drop wait_lock before the goto.
> +
> spin_unlock(&hb->lock);
>
> /* drops pi_state->pi_mutex.wait_lock */
> - ret = wake_futex_pi(uaddr, uval, pi_state);
> + ret = wake_futex_pi(uaddr, uval, pi_state, rt_waiter);
>
> put_pi_state(pi_state);
>
> @@ -1187,6 +1198,7 @@ int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
> return ret;
> }
>
> +do_uncontended:
> /*
> * We have no kernel internal state, i.e. no waiters in the
> * kernel. Waiters which are about to queue themselves are stuck
>
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