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From: Juan Quintela <quintela@redhat.com>
In-Reply-To: <55F00F83.2080708@redhat.com> (Paolo Bonzini's message of "Wed, 9
	Sep 2015 12:52:51 +0200")
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Date: Wed, 09 Sep 2015 13:01:03 +0200
Message-ID: <87a8sv6emo.fsf@neno.neno>
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Subject: Re: [Qemu-devel] [PATCH v7 1/5] cpu: Provide vcpu throttling
	interface
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, dgilbert@redhat.com, borntraeger@de.ibm.com, "Jason J. Herne" <jjherne@linux.vnet.ibm.com>, amit.shah@redhat.com, afaerber@suse.de

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.