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Date: Thu, 10 Oct 2024 16:05:14 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: John Stultz <jstultz@google.com>, LKML <linux-kernel@vger.kernel.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Joel Fernandes <joelaf@google.com>,
	Qais Yousef <qyousef@layalina.io>, Ingo Molnar <mingo@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Valentin Schneider <vschneid@redhat.com>,
	Ben Segall <bsegall@google.com>,
	Zimuzo Ezeozue <zezeozue@google.com>, Mel Gorman <mgorman@suse.de>,
	Will Deacon <will@kernel.org>, Waiman Long <longman@redhat.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Metin Kaya <Metin.Kaya@arm.com>,
	Xuewen Yan <xuewen.yan94@gmail.com>,
	K Prateek Nayak <kprateek.nayak@amd.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Daniel Lezcano <daniel.lezcano@linaro.org>, kernel-team@android.com,
	Connor O'Brien <connoro@google.com>
Subject: Re: [RESEND x3][PATCH v12 2/7] locking/mutex: Make mutex::wait_lock
 irq safe
Message-ID: <20241010140514.GJ17263@noisy.programming.kicks-ass.net>
References: <20241009235352.1614323-1-jstultz@google.com>
 <20241009235352.1614323-3-jstultz@google.com>
 <20241010100045.321ca4de@gandalf.local.home>
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In-Reply-To: <20241010100045.321ca4de@gandalf.local.home>

On Thu, Oct 10, 2024 at 10:00:45AM -0400, Steven Rostedt wrote:
> On Wed,  9 Oct 2024 16:53:35 -0700
> John Stultz <jstultz@google.com> wrote:
> 
> > From: Juri Lelli <juri.lelli@redhat.com>
> > 
> > mutex::wait_lock might be nested under rq->lock.
> > 
> > Make it irq safe then.
> 
> Can you expand on this please?
> 
> If the mutex:wait_lock might be taken under an rq->lock, doesn't that mean
> a mutex was taken under rq->lock? Or is it something internal?

Very similar to what we do for rt_mutex during PI; we'll want to look at
mutex_owner during the block chain walk, and holding wait_lock ensures
objects persistence.