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Date: Thu, 17 Sep 2026 17:38:31 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Suleiman Souhlal <suleiman@google.com>
Cc: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Darren Hart <dvhart@infradead.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	=?iso-8859-1?Q?Andr=E9?= Almeida <andrealmeid@igalia.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
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	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
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	zhidao su <soolaugust@gmail.com>, John Stultz <jstultz@google.com>,
	Qais Yousef <qyousef@google.com>, ssouhlal@freebsd.org
Subject: Re: [RFC PATCH 02/12] futex: Switch PI futex to use p->pi_futex_lock
 instead of p->pi_lock.
Message-ID: <20260917153831.GL4121339@noisy.programming.kicks-ass.net>
References: <20260917043339.2093426-1-suleiman@google.com>
 <20260917043339.2093426-3-suleiman@google.com>
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In-Reply-To: <20260917043339.2093426-3-suleiman@google.com>

On Thu, Sep 17, 2026 at 04:33:26AM +0000, Suleiman Souhlal wrote:
> Switch PI futexes to use p->pi_futex_lock instead of p->pi_lock.
> 
> When augmenting PING futexes with proxy execution, we get lock order
> inversions, due to the lock order being p->pi_lock -> mutex->wait_lock
> in the scheduler, but wait_lock -> p->pi_lock in futex code.
> 
> So move the futex code to use a new lock, p->pi_futex_lock, to
> protect p->pi_state_list and pi_state->owner.

This is of course horrible. Lets not do this.