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Re: [RFC PATCH 0/6] job: replace AioContext lock with job_mutex


From: Stefan Hajnoczi
Subject: Re: [RFC PATCH 0/6] job: replace AioContext lock with job_mutex
Date: Tue, 13 Jul 2021 14:10:22 +0100

On Mon, Jul 12, 2021 at 10:41:46AM +0200, Emanuele Giuseppe Esposito wrote:
> 
> 
> On 08/07/2021 15:04, Stefan Hajnoczi wrote:
> > On Thu, Jul 08, 2021 at 01:32:12PM +0200, Paolo Bonzini wrote:
> > > On 08/07/21 12:36, Stefan Hajnoczi wrote:
> > > > > What is very clear from this patch is that it
> > > > > is strictly related to the brdv_* and lower level calls, because
> > > > > they also internally check or even use the aiocontext lock.
> > > > > Therefore, in order to make it work, I temporarly added some
> > > > > aiocontext_acquire/release pair around the function that
> > > > > still assert for them or assume they are hold and temporarly
> > > > > unlock (unlock() - lock()).
> > > > 
> > > > Sounds like the issue is that this patch series assumes AioContext locks
> > > > are no longer required for calling the blk_*()/bdrv_*() APIs? That is
> > > > not the case yet, so you had to then add those aio_context_lock() calls
> > > > back in elsewhere. This approach introduces unnecessary risk. I think we
> > > > should wait until blk_*()/bdrv_*() no longer requires the caller to hold
> > > > the AioContext lock before applying this series.
> > > 
> > > In general I'm in favor of pushing the lock further down into smaller and
> > > smaller critical sections; it's a good approach to make further audits
> > > easier until it's "obvious" that the lock is unnecessary.  I haven't yet
> > > reviewed Emanuele's patches to see if this is what he's doing where he's
> > > adding the acquire/release calls, but that's my understanding of both his
> > > cover letter and your reply.
> > 
> > The problem is the unnecessary risk. We know what the goal is for
> > blk_*()/bdrv_*() but it's not quite there yet. Does making changes in
> > block jobs help solve the final issues with blk_*()/bdrv_*()?
> 
> Correct me if I am wrong, but it seems to me that the bdrv_*()/blk_*()
> operation mostly take care of building, modifying and walking the bds graph.
> So since graph nodes can have multiple AioContext, it makes sense that we
> have a lock when modifying the graph, right?
> 
> If so, we can simply try to replace the AioContext lock with a graph lock,
> or something like that. But I am not sure of this.

Block graph manipulation (all_bdrv_states and friends) requires the BQL.
It has always been this way.

This raises the question: if block graph manipulation is already under
the BQL and BlockDriver callbacks don't need the AioContext anymore, why
are aio_context_acquire() calls still needed in block jobs?

AIO_WAIT_WHILE() requires that AioContext is acquired according to its
documentation, but I'm not sure that's true anymore. Thread-safe/atomic
primitives are used by AIO_WAIT_WHILE(), so as long as the condition
being waited for is thread-safe too it should work without the
AioContext lock.

Back to my comment about unnecessary risk, pushing the lock down is a
strategy for exploring the problem, but I'm not sure those intermediate
commits need to be committed to qemu.git/master because of the time
required to review them and the risk of introducing (temporary) bugs.
Maybe there's a benefit to this patch series that I've missed?

Stefan

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