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Date:   Thu, 9 Sep 2021 14:45:24 +0100
From:   Will Deacon <will@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, boqun.feng@gmail.com,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Waiman Long <longman@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mike Galbraith <efault@gmx.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH 1/4] sched/wakeup: Strengthen
 current_save_and_set_rtlock_wait_state()
Message-ID: <20210909134524.GB9722@willie-the-truck>
References: <20210909105915.757320973@infradead.org>
 <20210909110203.767330253@infradead.org>
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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.

Will