linux-kernel - Re: [Bug, sched, 5.8-rc2]: PREEMPT kernels crashing in check_preempt

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Message-ID: <20200627183042.GK4817@hirez.programming.kicks-ass.net>
Date:   Sat, 27 Jun 2020 20:30:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [Bug, sched, 5.8-rc2]: PREEMPT kernels crashing in
 check_preempt_wakeup() running fsx on XFS

On Sat, Jun 27, 2020 at 08:32:54AM +1000, Dave Chinner wrote:
> Observation from the outside:
> 
> "However I'm having trouble convincing myself that's actually
> possible on x86_64.... "

Using the weaker rules of LKMM (as relevant to Power) I could in fact
make it happen, the 'problem' is that it's being observed on the much
stronger x86_64.

So possibly I did overlook a more 'sensible' scenario, but I'm pretty
confident the problem holds as it fully explains the failure mode.

> This scheduler code has fallen off a really high ledge on the memory
> barrier cliff, hasn't it?

Just a wee bit.. I did need pen and paper and a fair amount of
scribbling for this one.

> Having looked at this code over the past 24 hours and the recent
> history, I know that understanding it - let alone debugging and
> fixing problem in it - is way beyond my capabilities.  And I say
> that as an experienced kernel developer with a pretty good grasp of
> concurrent programming and a record of implementing a fair number of
> non-trivial lockless algorithms over the years....

All in the name of making it go fast, I suppose. It used to be much
simpler... like much of the kernel.

The biggest problem I had with this thing was that the reproduction case
we had (Paul's rcutorture) wouldn't readily trigger on my machines
(altough it did, but at a much lower rate, just twice in a week's worth
of runtime).

Also; I'm sure you can spot a problem in the I/O layer much faster than
I possibly could :-)

Anyway, let me know if you still observe any problems.