Re: [Qemu-devel] [RFC] aio: add aio_context_acquire() and aio_context_release()

[Paolo Bonzini <pbonzini@redhat.com>]

Il 30/08/2013 11:22, Stefan Hajnoczi ha scritto:
> On Thu, Aug 29, 2013 at 10:26:31AM +0200, Paolo Bonzini wrote:
>> Il 27/08/2013 16:39, Stefan Hajnoczi ha scritto:
>>> +void aio_context_acquire(AioContext *ctx)
>>> +{
>>> +    qemu_mutex_lock(&ctx->acquire_lock);
>>> +    while (ctx->owner) {
>>> +        assert(!qemu_thread_is_self(ctx->owner));
>>> +        aio_notify(ctx); /* kick current owner */
>>> +        qemu_cond_wait(&ctx->acquire_cond, &ctx->acquire_lock);
>>> +    }
>>> +    qemu_thread_get_self(&ctx->owner_thread);
>>> +    ctx->owner = &ctx->owner_thread;
>>> +    qemu_mutex_unlock(&ctx->acquire_lock);
>>> +}
>>> +
>>> +void aio_context_release(AioContext *ctx)
>>> +{
>>> +    qemu_mutex_lock(&ctx->acquire_lock);
>>> +    assert(ctx->owner && qemu_thread_is_self(ctx->owner));
>>> +    ctx->owner = NULL;
>>> +    qemu_cond_signal(&ctx->acquire_cond);
>>> +    qemu_mutex_unlock(&ctx->acquire_lock);
>>> +}
>>
>> Thinking more about it, there is a risk of busy waiting here if one
>> thread releases the AioContext and tries to acquire it again (as in the
>> common case of one thread doing acquire/poll/release in a loop).  It
>> would only work if mutexes guarantee some level of fairness.
> 
> You are right.  I wrote a test that showed there is no fairness.  For
> some reason I thought the condvar would provide fairness.
> 
>> If you implement recursive acquisition, however, you can make aio_poll
>> acquire the context up until just before it invokes ppoll, and then
>> again after it comes back from the ppoll.  The two acquire/release pair
>> will be no-ops if called during "synchronous" I/O such as
>>
>>   /* Another thread */
>>   aio_context_acquire(ctx);
>>   bdrv_read(bs, 0x1000, buf, 1);
>>   aio_context_release(ctx);
>>
>> Yet they will do the right thing when called from the event loop thread.
>>
>> (where the bdrv_read can actually be something more complicated such as
>> a live snapshot or, in general, anything involving bdrv_drain_all).
> 
> This doesn't guarantee fairness either, right?

Yes, but the non-zero timeout of ppoll would in practice guarantee it.
The problem happens only when the release and acquire are very close in
time, which shouldn't happen if the ppoll is done released.

> With your approach another thread can squeeze in when ppoll(2) is
> returning so newer fd activity can be processed before we processed
> *before* older activity.  Not sure out-of-order callbacks are a problem
> but it can happen since we don't have fairness.

I think this should not happen.  The other thread would rerun ppoll(2).
 Since poll/ppoll are level-triggered, you could have some flags
processed twice.  But this is not a problem, we had the same bug with
iothread and qemu_aio_wait and we should have fixed all occurrences.

> But at least this way other threads can acquire the AioContext while
> ppoll(2) is blocked without racing against each other for the acquire
> lock.

Yes, that's also true.

Paolo