Re: [Qemu-devel] [PATCH 0/3] qemu-timer: make QEMUTimer functions thread-safe

[Paolo Bonzini]

Il 05/07/2013 19:51, Jan Kiszka ha scritto:
> On 2013-07-05 14:39, Stefan Hajnoczi wrote:
>> This series makes the following functions thread-safe:
>>
>>   qemu_mod_timer_ns()
>>   qemu_mod_timer()
>>   qemu_del_timer()
>>   qemu_timer_pending()
>>
>> The following were already thread-safe:
>>
>>   qemu_free_timer()
>>   qemu_new_timer()
>>   qemu_timer_expired()
>>
>> Now it is possible to use QEMUTimer outside the QEMU global mutex.  Timer
>> callbacks are still invoked from the main loop.  If a thread wishes to run
>> timer callbacks it must use a thread-safe QEMUBH (which Ping Fan Liu is 
>> working
>> on).
> 
> What do you mean with this? We need main-loop independent timers for any
> task that depends on timely alarm delivery. Do your patches keep this in
> mind as well?

These are orthogonal issues.  Stefan's usecase does not need timely
delivery.

>> Note that host_clock is not thread-safe because it keeps state and invokes
>> reset notifiers.  Device emulation threads mostly care about vm_clock, so 
>> this
>> is not a problem.
> 
> I suppose you know that vm_clock cannot be read outside BQL yet due to
> timers_state and, under TCG, icount. Any ideas regarding this already? I
> didn't have to solve that problem so far as I only need CLOCK_REALTIME
> outside BQL.

I was thinking of a seqlock.  It should be quite cheap, since there
would be hardly any contention.

No ideas about host_clock's notifiers.

Paolo