linux-kernel - Re: Possible reproduction of CSD locking issue

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Date:   Wed, 26 Jan 2022 09:31:17 -0600
From:   Corey Minyard <minyard@....org>
To:     Juergen Gross <jgross@...e.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Possible reproduction of CSD locking issue

On Wed, Jan 26, 2022 at 03:51:36PM +0100, Juergen Gross wrote:
> On 26.01.22 14:56, Corey Minyard wrote:
> > On Wed, Jan 26, 2022 at 07:08:22AM +0100, Juergen Gross wrote:

snip..

> > 
> >   csd: cnt(63d8e1f): 0003->0037 queue
> >   csd: cnt(63d8e20): 0003->0037 ipi
> >   csd: cnt(63d8e21): 0003->0037 ping
> > 
> > In __smp_call_single_queue_debug CPU 3 sends another message to
> > CPU 55 and sends an IPI.  But there should be a pinged entry
> > after this.
> > 
> >   csd: cnt(63d8e22): 0003->0037 queue
> >   csd: cnt(63d8e23): 0003->0037 noipi
> 
> This is interesting. Those are 5 consecutive entries without any
> missing in between (see the counter values). Could it be that after
> the ping there was an interrupt and the code was re-entered for
> sending another IPI? This would clearly result in a hang as seen.

Since preempt is enabled, wouldn't it eventually come back to the first
thread and send the IPI?  Unless CPU 3 is stuck in an interrupt or
interrupt storm.

-corey