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Date:   Tue, 1 Aug 2023 14:32:18 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Guenter Roeck <linux@roeck-us.net>, Roy Hopkins <rhopkins@suse.de>,
        Joel Fernandes <joel@joelfernandes.org>,
        Pavel Machek <pavel@denx.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, patches@lists.linux.dev,
        linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        akpm@linux-foundation.org, shuah@kernel.org, patches@kernelci.org,
        lkft-triage@lists.linaro.org, jonathanh@nvidia.com,
        f.fainelli@gmail.com, sudipm.mukherjee@gmail.com,
        srw@sladewatkins.net, rwarsow@gmx.de, conor@kernel.org,
        rcu@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Marco Elver <elver@google.com>
Subject: Re: scheduler problems in -next (was: Re: [PATCH 6.4 000/227]
 6.4.7-rc1 review)
Message-ID: <9154ee27-6f38-4efe-9391-ef626cdc2ff4@paulmck-laptop>
Reply-To: paulmck@kernel.org
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On Tue, Aug 01, 2023 at 09:08:52PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 01, 2023 at 10:32:45AM -0700, Guenter Roeck wrote:
> > On 7/31/23 14:15, Peter Zijlstra wrote:
> > > On Mon, Jul 31, 2023 at 09:34:29AM -0700, Guenter Roeck wrote:
> > > > > Ha!, I was poking around the same thing. My hack below seems to (so far,
> > > > > <20 boots) help things.
> > > > > 
> > > > 
> > > > So, dumb question:
> > > > How comes this bisects to "sched/fair: Remove sched_feat(START_DEBIT)" ?
> > > 
> > > That commit changes the timings of things; dumb luck otherwise.
> > 
> > Kind of scary. So I only experienced the problem because the START_DEBIT patch
> > happened to be queued roughly at the same time, and it might otherwise have
> > found its way unnoticed into the upstream kernel. That makes me wonder if this
> > or other similar patches may uncover similar problems elsewhere in the kernel
> > (i.e., either hide new or existing race conditions or expose existing ones).
> > 
> > This in turn makes me wonder if it would be possible to define a test which
> > would uncover such problems without the START_DEBIT patch. Any idea ?
> 
> IIRC some of the thread sanitizers use breakpoints to inject random
> sleeps, specifically to tickle races.

I have heard of are some of these, arguably including KCSAN, but they
would have a tough time on this one.

They would have to inject many milliseconds between the check of
->kthread_ptr in synchronize_rcu_tasks_generic() and that mutex_lock()
in rcu_tasks_one_gp().  Plus this window only occurs during boot shortly
before init is spawned.

On the other hand, randomly injecting delay just before acquiring each
lock would cover this case.  But such a sanitzer would still only get
one shot per boot of the kernel for this particular bug.

							Thanx, Paul