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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Message-ID: <20190110110839.7daeef3d@gandalf.local.home>
Date:   Thu, 10 Jan 2019 11:08:39 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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 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?

-- 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
>