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

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Date:   Wed, 9 Jan 2019 20:38:51 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        rostedt <rostedt@...dmis.org>
Subject: Possible use of RCU while in extended QS: idle vs RCU read-side in
 interrupt vs rcu_eqs_exit

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.

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