Messages in this thread | | | Subject | Re: [PATCH] sched: fix rq lock recursion issue | From | Satya Durga Srinivasu Prabhala <> | Date | Tue, 5 Jul 2022 20:46:54 -0700 |
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On 7/1/22 1:33 AM, 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. Sorry about that. I'm cross checking further on ways to repro issue easily. >> 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. > Thanks for explanation. Will cross check further on these points. >>> -010 |spin_bug(lock = ???, msg = ???) >>> -011 |debug_spin_lock_before(inline) >>> -011 |do_raw_spin_lock(lock = 0xFFFFFF89323BB600) >>> -012 |_raw_spin_lock(inline) >>> -012 |raw_spin_rq_lock_nested(inline) >>> -012 |raw_spin_rq_lock(inline) >>> -012 |task_rq_lock(p = 0xFFFFFF88CFF1DA00, rf = 0xFFFFFFC03707BBE8) >>> -013 |__set_cpus_allowed_ptr(inline) >>> -013 |migrate_enable() >>> -014 |trace_call_bpf(call = ?, ctx = 0xFFFFFFFDEF954600) >>> -015 |perf_trace_run_bpf_submit(inline) >>> -015 |perf_trace_sched_switch(__data = 0xFFFFFFE82CF0BCB8, preempt = FALSE, prev = ?, next = ?) >>> -016 |__traceiter_sched_switch(inline) >>> -016 |trace_sched_switch(inline) >>> -016 |__schedule(sched_mode = ?) >>> -017 |schedule() >>> -018 |arch_local_save_flags(inline) >>> -018 |arch_irqs_disabled(inline) >>> -018 |__raw_spin_lock_irq(inline) >>> -018 |_raw_spin_lock_irq(inline) >>> -018 |worker_thread(__worker = 0xFFFFFF88CE251300) >>> -019 |kthread(_create = 0xFFFFFF88730A5A80) >>> -020 |ret_from_fork(asm) > This doesn't clarify much. Please explain how things got to be > unbalanced, don't ever just make a problem dissapear like this without > understanding what the root cause is, that'll just get your reputation > sullied. Agreed, thanks for the comments and suggestion. Yes, I'm trying to cross check further and find ways to repro the issue. Will get back once I find a better way to handle the issue. I should have just tried to get comments/feedback on the issue first instead proposing fix. Lesson learnt :)
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