linux-kernel - Re: [PATCH] sched: fix rq lock recursion issue

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Date:   Fri, 1 Jul 2022 11:54:53 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Satya Durga Srinivasu Prabhala <quic_satyap@...cinc.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix rq lock recursion issue

On Thu, 30 Jun 2022 18:37:04 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> > 
> > Fix the issue by switching to preempt_enable/disable() for non-RT
> > Kernels.
> > 
> > -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)  
> 
> trace_sched_switch() disables preemption.
> 
> So how is this a fix?

Let me rephrase my question.

As trace_sched_switch() disables preemption, why is trace_call_bpf()
calling migrate_disable()?

Looks like you could modify the code to include a __bpf_prog_run_array()
that skips the migrate_disable(). You even have a "cant_sleep()" call in
trace_call_bpf().

-- Steve