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Date:   Sat, 15 Oct 2022 17:08:14 +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: <Y0rM3gHajrNdPcyi@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:
> @@ -965,7 +1026,10 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
>  	if (owner & MUTEX_FLAG_HANDOFF)
>  		__mutex_handoff(lock, next);
>  
> -	preempt_disable();
> +	preempt_disable(); /* XXX connoro: why disable preemption here? */
> +#ifdef CONFIG_PROXY_EXEC
> +	raw_spin_unlock(&current->blocked_lock);
> +#endif
>  	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);

Because if we wouldn't have preemption disabled it would preempt here,
before the wakeup:

>  	wake_up_q(&wake_q);

And you'd be stuck with a priority inversion.