Re: [PATCH] sched: fix rq lock recursion issue

[Satya Durga Srinivasu Prabhala]

On 7/1/22 4:48 AM, Qais Yousef wrote:
> On 07/01/22 10:33, Peter Zijlstra wrote:
> > On Thu, Jun 30, 2022 at 10:53:10PM +0100, Qais Yousef wrote:
> > > Hi Satya
> > >
> > > On 06/24/22 00:42, Satya Durga Srinivasu Prabhala wrote:
> > > > Below recursion is observed in a rare scenario where __schedule()
> > > > takes rq lock, at around same time task's affinity is being changed,
> > > > bpf function for tracing sched_switch calls migrate_enabled(),
> > > > checks for affinity change (cpus_ptr != cpus_mask) lands into
> > > > __set_cpus_allowed_ptr which tries acquire rq lock and causing the
> > > > recursion bug.
> > > >
> > > > Fix the issue by switching to preempt_enable/disable() for non-RT
> > > > Kernels.
> > > Interesting bug. Thanks for the report. Unfortunately I can't see this being
> > > a fix as it just limits the bug visibility to PREEMPT_RT kernels, but won't fix
> > > anything, no? ie: Kernels compiled with PREEMPT_RT will still hit this failure.
> > Worse, there's !RT stuff that grew to rely on the preemptible migrate
> > disable stuff, so this actively breaks things.
> >
> > > I'm curious how the race with set affinity is happening. I would have thought
> > > user space would get blocked as __schedule() will hold the rq lock.
> > >
> > > Do you have more details on that?
> > Yeah, I'm not seeing how this works either, in order for
> > migrate_enable() to actually call __set_cpus_allowed_ptr(), it needs to
> > have done migrate_disable() *before* schedule, schedule() will then have
> > to call migrate_disable_swich(), and *then* migrate_enable() does this.
> >
> > However, if things are nicely balanced (as they should be), then
> > trace_call_bpf() using migrate_disable()/migrate_enable() should never
> > hit this path.
> >
> > If, OTOH, migrate_disable() was called prior to schedule() and we did do
> > migrate_disable_switch(), then it should be impossible for the
> > tracepoint/bpf stuff to reach p->migration_disabled == 0.
> I think it's worth to confirm which kernel Satya is on too. If it's GKI, then
> worth checking first this is actually reproducible on/applicable to mainline.
We are seeing the issue on 5.15 GKI Kernel. On older Kernels, like 5.10 
Kernel
migrate_disable/enable() end-up calling preempt_disable/enable(). I will 
cross
check further on comments and inputs I received so far.

> Cheers
>
> --
> Qais Yousef