Messages in this thread | | | Date | Fri, 1 Jul 2022 10:33:01 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: fix rq lock recursion issue |
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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.
> > -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.
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