Messages in this thread | | | Subject | Re: [PATCH] sched: fix rq lock recursion issue | From | Satya Durga Srinivasu Prabhala <> | Date | Tue, 5 Jul 2022 20:45:11 -0700 |
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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
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