linux-kernel - Re: [PATCH 1/4] sched/wakeup: Strengthen current_save_and_set_rtlock_wait

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Date:   Thu, 9 Sep 2021 16:27:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     tglx@...utronix.de, boqun.feng@...il.com,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <longman@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mike Galbraith <efault@....de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 1/4] sched/wakeup: Strengthen
 current_save_and_set_rtlock_wait_state()

On Thu, Sep 09, 2021 at 02:45:24PM +0100, Will Deacon wrote:
> On Thu, Sep 09, 2021 at 12:59:16PM +0200, Peter Zijlstra wrote:
> > While looking at current_save_and_set_rtlock_wait_state() I'm thinking
> > it really ought to use smp_store_mb(), because something like:
> > 
> > 	current_save_and_set_rtlock_wait_state();
> > 	for (;;) {
> > 		if (try_lock())
> > 			break;
> > 
> > 		raw_spin_unlock_irq(&lock->wait_lock);
> > 		schedule();
> > 		raw_spin_lock_irq(&lock->wait_lock);
> > 
> > 		set_current_state(TASK_RTLOCK_WAIT);
> > 	}
> > 	current_restore_rtlock_saved_state();
> > 
> > which is the advertised usage in the comment, is actually broken,
> > since trylock() will only need a load-acquire in general and that
> > could be re-ordered against the state store, which could lead to a
> > missed wakeup -> BAD (tm).
> 
> Why doesn't the UNLOCK of pi_lock in current_save_and_set_rtlock_wait_state()
> order the state change before the successful try_lock? I'm just struggling
> to envisage how this actually goes wrong.

Moo yes, so the earlier changelog I wrote was something like:

	current_save_and_set_rtlock_wait_state();
	for (;;) {
		if (try_lock())
			break;

		raw_spin_unlock_irq(&lock->wait_lock);
		if (!cond)
			schedule();
		raw_spin_lock_irq(&lock->wait_lock);

		set_current_state(TASK_RTLOCK_WAIT);
	}
	current_restore_rtlock_saved_state();

which is more what the code looks like before these patches, and in that
case the @cond load can be lifted before __state.

It all sorta works in the current application because most things are
serialized by ->wait_lock, but given the 'normal' wait pattern I got
highly suspicious of there not being a full barrier around.