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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

(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. :)

Paolo