Re: [Qemu-devel] [PATCH v7 1/5] cpu: Provide vcpu throttling interface

[Juan Quintela <quintela@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 09/09/2015 12:41, Juan Quintela wrote:
>>> > +    qemu_mutex_unlock_iothread();
>>> > +    atomic_set(&cpu->throttle_thread_scheduled, 0);
>>> > +    g_usleep(sleeptime_ns / 1000); /* Convert ns to us for usleep call */
>>> > +    qemu_mutex_lock_iothread();
>> 
>> Why is this thread safe?
>> 
>> qemu_mutex_lock_iothread() is protecting (at least) cpu_work_first on
>> each cpu.  How can we be sure that _nothing_ will change that while we
>> are waiting?
>
> You only have to be sure that the queued work list remains consistent;
> not that nothing changes.


But nothing else is protected by the iothread?  That is the part that I
can't see.

> (BTW, there is a queued patch that moves the queued work list to its own
> mutex, and indeed it releases that mutex while calling the work function).


>> A fast look through the tree don't show anything that
>> runs here that drops the lock.
>
> Actually, the existing implementation of throttling does. :)

See, that happens when you search in a modified tree O:-)

Thanks for the fast answer.

Later, Juan.