linux-kernel - Re: [patch 03/50] sched: Prepare for RT sleeping spin/rwlocks

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Message-ID: <20210715092703.GI2725@worktop.programming.kicks-ass.net>
Date:   Thu, 15 Jul 2021 11:27:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [patch 03/50] sched: Prepare for RT sleeping spin/rwlocks

On Thu, Jul 15, 2021 at 12:20:28AM +0100, Valentin Schneider wrote:
> Hi,
> 
> On 13/07/21 17:10, Thomas Gleixner wrote:
> > From: Thomas Gleixner <tglx@...utronix.de>
> >
> > Waiting for spinlocks and rwlocks on non RT enabled kernels is task::state
> > preserving. Any wakeup which matches the state is valid.
> >
> > RT enabled kernels substitutes them with 'sleeping' spinlocks. This creates
> > an issue vs. task::state.
> >
> > In order to block on the lock the task has to overwrite task::state and a
> > consecutive wakeup issued by the unlocker sets the state back to
> > TASK_RUNNING. As a consequence the task loses the state which was set
> > before the lock acquire and also any regular wakeup targeted at the task
> > while it is blocked on the lock.
> >
> 
> I'm not sure I get this for spinlocks - p->__state != TASK_RUNNING means
> task is stopped (or about to be), IMO that doesn't go with spinning. I was
> thinking perhaps ptrace could be an issue, but I don't have a clear picture
> on that either. What am I missing?

spinlocks will become rtmutex. They're going to clobber __state by
virtue of a nested block.