Re: [Qemu-devel] [PATCH v4 2/3] qemu-timer: make qemu_timer_mod_ns() and qemu_timer_del() thread-safe

[Alex Bligh]

On 30 Sep 2013, at 13:45, Mike Day wrote:

> I've applied this set to Paolo's rcu tree - I see a couple of routines
> that appear to need the active_timers_lock:
> 
> (line 137 of qemu-timer.c in my tree)
> void qemu_clock_notify(QEMUClockType type)
> {
>    QEMUTimerList *timer_list;
>    QEMUClock *clock = qemu_clock_ptr(type);
>    QLIST_FOREACH(timer_list, &clock->timerlists, list) {
>        timerlist_notify(timer_list);
>    }
> }
> 
> (line 228 of qemu-timer.c in my tree)
> int64_t qemu_clock_deadline_ns_all(QEMUClockType type)
> {
>    int64_t deadline = -1;
>    QEMUTimerList *timer_list;
>    QEMUClock *clock = qemu_clock_ptr(type);
>    QLIST_FOREACH(timer_list, &clock->timerlists, list) {
>        deadline = qemu_soonest_timeout(deadline,
>                                        timerlist_deadline_ns(timer_list));
>    }
>    return deadline;
> }
> 
> I think these functions are always called now with the BQL held, so I
> wonder if they are good candidates for RCU? 

These routines iterate through the list of timerlists held by
a clock.

They do not iterate through the list of active timers in a timer
list. I believe the latter is what active_timers_lock protects.

The list of timers attached to a clock is only modified when timers
are created and deleted which is (currently) under the BQL.

-- 
Alex Bligh