Re: [PATCH v3 7/7] locking/rtmutex: Acquire the hb lock via trylock after wait-proxylock.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2023-09-11 16:11:35 [+0200], Peter Zijlstra wrote:
> Aside from this being just plain gross, this also destroys determinism
> of futex_pi, which completely defeats the purpose of the whole thing.

No objections here.

> 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.

Sounds harmless. I sure something will be pop up in a few years and we
will look back this ;)

> Would not something like the below work instead?

The following is what I ended up with. It adds the `struct' keywords to
wake_futex_pi() and accounts for rt_mutex_post_schedule() in
futex_lock_pi() (assuming you stuff this at the end.

Comments follow…

diff --git a/kernel/futex/pi.c b/kernel/futex/pi.c
index f8e65b27d9d6b..d8866278e92ff 100644
--- a/kernel/futex/pi.c
+++ b/kernel/futex/pi.c
@@ -611,29 +611,16 @@ 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,
+			 struct 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;
 
 	/*
@@ -1046,15 +1033,18 @@ 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;
@@ -1063,6 +1053,12 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
 	 * Waiter is unqueued.
 	 */
 	rt_mutex_post_schedule();
+
+	/*
+	 * 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
@@ -1143,6 +1139,7 @@ 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)
@@ -1158,19 +1155,34 @@ 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;
+
 		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);
 
@@ -1198,6 +1210,7 @@ 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

Sebastian