linux-kernel - Re: Possible use of RCU while in extended QS: idle vs RCU read-side in interrupt vs rcu_eqs

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Date:   Thu, 10 Jan 2019 11:44:13 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: Possible use of RCU while in extended QS: idle vs RCU read-side
 in interrupt vs rcu_eqs_exit

----- On Jan 10, 2019, at 8:08 AM, rostedt rostedt@...dmis.org wrote:

> On Wed, 9 Jan 2019 20:38:51 -0500 (EST)
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>> Hi Paul,
>> 
>> I've had a user report that trace_sched_waking() appears to be
>> invoked while !rcu_is_watching() in some situation, so I started
>> digging into the scheduler idle code.
> 
> I'm wondering if this isn't a bug. Do you have the backtrace for where
> trace_sched_waking() was called without rcu watching?

I strongly suspect a bug as well. I'm awaiting a reproducer from the
user whom reported this issue so I can add a WARN_ON_ONCE(!rcu_is_watching())
in the scheduler code near trace_sched_waking() and gather a backtrace.

It still has to be confirmed, but I suspect this have been triggered
within a HyperV guest. It may therefore be related to a virtualized environment.

I'll try to ask more specifically on which environment this was encountered.

Thanks,

Mathieu

> 
> -- Steve
> 
>> 
>> It appears that interrupts are re-enabled before rcu_eqs_exit() is
>> invoked when exiting idle code from the scheduler.
>> 
>> I wonder what happens if an interrupt handler (including scheduler code)
>> happens to issue a RCU read-side critical section before rcu_eqs_exit()
>> is called ? Is there some code on interrupt entry that ensures rcu eqs
>> state is exited in such scenario ?
>> 
>> Thanks,
>> 
>> Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com