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

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Date:   Thu, 15 Jul 2021 15:08:36 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 15/07/21 11:27, Peter Zijlstra wrote:
> 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.

I wasn't expecting there to be any task taking spinlocks with state !=
TASK_RUNNING, but I just didn't know where to look.

For instance do_wait() sets current to TASK_INTERRUPTIBLE and can then faff
around with some sighand locks before eventually calling into schedule(),
so clearly that one would be affected by the clobbering.

The more you know...