Messages in this thread | | | Date | Fri, 1 Jul 2022 12:48:46 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH] sched: fix rq lock recursion issue |
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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.
Cheers
-- Qais Yousef
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