Messages in this thread | | | Date | Wed, 9 Jan 2019 20:13:28 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Possible use of RCU while in extended QS: idle vs RCU read-side in interrupt vs rcu_eqs_exit |
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On Wed, Jan 09, 2019 at 08:38:51PM -0500, Mathieu Desnoyers 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. > > 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 ?
Interrupt handlers are supposed to invoke irq_enter(), which will in turn invoke rcu_irq_enter(), which should take care of things.
However, there are cases where a given architecture knows that a given interrupt handler does not contain RCU readers, and in this case, the architecture might omit the rcu_irq_enter() or maybe even the whole irq_enter(). And then it is all fun and games until someone adds an RCU read-side critical section. ;-)
Thanx, Paul
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