Messages in this thread | | | Date | Thu, 10 Jan 2019 12:23:46 -0500 | From | Steven Rostedt <> | 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 Thu, 10 Jan 2019 12:11:55 -0500 (EST) Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> ----- On Jan 10, 2019, at 8:44 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote: > > > ----- On Jan 10, 2019, at 8:08 AM, rostedt rostedt@goodmis.org wrote: > > > >> On Wed, 9 Jan 2019 20:38:51 -0500 (EST) > >> Mathieu Desnoyers <mathieu.desnoyers@efficios.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. > > So it ends up it happens directly on hardware on a Linux laptop. Here is > the stacktrace: > > vmlinux!try_to_wake_up > vmlinux!default_wake_function > vmlinux!pollwake > vmlinux!__wake_up_common > vmlinux!__wake_up_common_lock > vmlinux!__wake_up > vmlinux!perf_event_wakeup > vmlinux!perf_pending_event > vmlinux!irq_work_run_list > vmlinux!irq_work_run
> vmlinux!smp_irq_work_iterrupt
The first thing this calls is ipi_entering_ack_irq(), which calls irq_enter(), which calls rcu_irq_enter().
So why isn't rcu watching here?
-- Steve
> vmlinux!irq_work_interrupt > vmlinux!finish_task_switch > vmlinux!__schedule > vmlinux!schedule_idle > vmlinux!do_idle > vmlinux!cpu_startup_entry > vmlinux!start_secondary > vmlinux!secondary_startup_64 > > Does it raise any red flag ? > > Thanks, > > Mathieu > > > > > 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 >
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