linux-kernel - Re: [PATCH RT 1/2] tasklet: Address a race resulting in double-enqueue

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Message-ID: <4ccbc655514eb338a88bea28584ab5611e9a2b88.camel@kernel.org>
Date:   Tue, 09 Jun 2020 11:57:31 -0500
From:   Tom Zanussi <zanussi@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Ramon Fried <rfried.dev@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Clark Williams <williams@...hat.com>,
        Zhang Xiao <xiao.zhang@...driver.com>
Subject: Re: [PATCH RT 1/2] tasklet: Address a race resulting in
 double-enqueue

Hi Sebastian,

On Tue, 2020-06-09 at 18:34 +0200, Sebastian Andrzej Siewior wrote:
> On 2020-06-09 11:17:53 [-0500], Tom Zanussi wrote:
> > Hi Sebastian,
> 
> Hi Tom,
> 
> > I did find a problem with the patch when configured as !SMP since
> > in
> > that case the RUN flag is never set (will send a patch for that
> > shortly), but that wouldn't be the case here.
> 
> How?
> 

My test machine with !SMP and !RT doesn't boot, and in that case we
have:

#define tasklet_trylock(t) 1
#define tasklet_tryunlock(t)    1

instead of setting/clearing the RUN flag.

So the cmpxchg with RUN+CHAIN can never work and we hit the loop.

> > #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
> > static inline int tasklet_trylock(struct tasklet_struct *t)
> > {
> >         return !test_and_set_bit(TASKLET_STATE_RUN, &(t)->state);
> > }
> 
> I can't tell from the backtrace if he runs with RT or without but I
> assumed RT. But yes, for !SMP && !RT it would explain it.
> 

Yeah, for me !SMP and RT works, but !SMP and !RT doesn't.

I had assumed he was talking about the samely configured kernel, but
apparently it's not.

Tom

> > It would help to be able to reproduce it, but I haven't been able
> > to
> > yet.
> > 
> > Tom
> > 
> 
> Sebastian