linux-kernel - Re: [PATCH v3 00/14] Generalized Priority Inheritance via Proxy Execution v3

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Date:   Sat, 22 Apr 2023 13:54:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Stultz <jstultz@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Qais Yousef <qyousef@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Zimuzo Ezeozue <zezeozue@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v3 00/14] Generalized Priority Inheritance via Proxy
 Execution v3



Anyway... tglx and bigeasy just pointed me at a problem with
pi_blocked_on vs sched_submit_work(), where if that function were to
block (specifically use rt-mutex), then we get:

	__rt_mutex_slowlock()
	  task_blocks_on_rt_mutex()
	    current->pi_blocked_on = waiter;
	  rt_mutex_slowlock_block()
	    schedule()
	      sched_submit_work()
	        ...
		  rt_mutex_lock()
		    ...
		      current->pi_blocked_on = waiter; // <--- OOPS

And AFAICT we have the exact same problem with the current DEBUG_MUTEXES
->blocked_on state which in this patch-set is generalized and relied
upon more heavily.