linux-kernel - Re: [PATCH 0/6] locking/rtmutex: Avoid PI state recursion through sched_submit

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Message-ID: <20230816152231.GD982867@hirez.programming.kicks-ass.net>
Date:   Wed, 16 Aug 2023 17:22:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bsegall@...gle.com, boqun.feng@...il.com, swood@...hat.com,
        bristot@...hat.com, dietmar.eggemann@....com, mingo@...hat.com,
        jstultz@...gle.com, juri.lelli@...hat.com, mgorman@...e.de,
        rostedt@...dmis.org, vschneid@...hat.com,
        vincent.guittot@...aro.org, longman@...hat.com, will@...nel.org
Subject: Re: [PATCH 0/6] locking/rtmutex: Avoid PI state recursion through
 sched_submit_work()

On Wed, Aug 16, 2023 at 04:58:18PM +0200, Peter Zijlstra wrote:

> I've ended up with the below, but it is quite horrible.. but let me go
> stare at the futex wreckage before trying to clean things up.

OK, I think the below covers the simple case, now lets see if I can make
sense of futex_wait_requeue_pi()... :/

---
--- a/kernel/futex/pi.c
+++ b/kernel/futex/pi.c
@@ -1002,6 +1002,12 @@ int futex_lock_pi(u32 __user *uaddr, uns
 		goto no_block;
 	}
 
+	/*
+	 * Must be done before we enqueue the waiter, here is unfortunately
+	 * under the hb lock, but that *should* work.
+	 */
+	rt_mutex_pre_schedule();
+
 	rt_mutex_init_waiter(&rt_waiter);
 
 	/*
@@ -1052,6 +1058,10 @@ int futex_lock_pi(u32 __user *uaddr, uns
 	if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter))
 		ret = 0;
 
+	/*
+	 * Waiter is unqueued.
+	 */
+	rt_mutex_post_schedule();
 no_block:
 	/*
 	 * Fixup the pi_state owner and possibly acquire the lock if we