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Date:   Sat, 15 Oct 2022 17:10:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Connor O'Brien <connoro@google.com>
Cc:     linux-kernel@vger.kernel.org, kernel-team@android.com,
        John Stultz <jstultz@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [RFC PATCH 07/11] sched: Add proxy execution
Message-ID: <Y0rNSMg+KmVnZ1oe@hirez.programming.kicks-ass.net>
References: <20221003214501.2050087-1-connoro@google.com>
 <20221003214501.2050087-8-connoro@google.com>
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On Mon, Oct 03, 2022 at 09:44:57PM +0000, Connor O'Brien wrote:
> @@ -703,15 +707,28 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
>  			break;
>  
>  		if (first) {
> +			bool acquired;
> +
> +			/*
> +			 * XXX connoro: mutex_optimistic_spin() can schedule, so

I'm thinking it might be better to remove that schedule point for
PROXY_EXEC instead of doing this.

Getting preempted right after acquiring a lock is less of a problem with
PE on than it is without it.

> +			 * we need to release these locks before calling it.
> +			 * This needs refactoring though b/c currently we take
> +			 * the locks earlier than necessary when proxy exec is
> +			 * disabled and release them unnecessarily when it's
> +			 * enabled. At a minimum, need to verify that releasing
> +			 * blocked_lock here doesn't create any races.
> +			 */
> +			raw_spin_unlock(&current->blocked_lock);
> +			raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
>  			trace_contention_begin(lock, LCB_F_MUTEX | LCB_F_SPIN);
> -			if (mutex_optimistic_spin(lock, ww_ctx, &waiter))
> +			acquired = mutex_optimistic_spin(lock, ww_ctx, &waiter);
> +			raw_spin_lock_irqsave(&lock->wait_lock, flags);
> +			raw_spin_lock(&current->blocked_lock);
> +			if (acquired)
>  				break;
>  			trace_contention_begin(lock, LCB_F_MUTEX);
>  		}