On Tue, Jul 13, 2021 at 06:18:39PM +0300, Vladimir Sementsov-Ogievskiy wrote: > 13.07.2021 16:10, Stefan Hajnoczi wrote: > > 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 > > I don't believe that block drivers are thread-safe now. They have some > mutexes.. But who make an audit of thread-safety?
Emanuele :) FWIW I took a look at the stream, mirror, and backup jobs today and couldn't find anything that's unsafe after this series. I was expecting to find issues but I think Emanuele and Paolo have taken care of them. > > 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. > > I agree. Add my bit of criticism: > > What I dislike about the whole thread-safe update you do: > > 1. There is no proof of concept - some good example of multiqueue, or > something that uses mutliple threads and shows good performance. Something > that works, and shows where are we going to and why it is good. That may be > draft patches with a lot of "FIXME" and "TODO", but working. For now I feel > that I've spent my time to reviewing and proving to myself thread-safety of > two previous thread-safe series, but I don't have a hope to see a benefit of > it in the near future.. The multi-queue block layer should improve performance in cases where the bottleneck is a single IOThread. It will allow users to assign more than one IOThread. But I think the bigger impact of this work will be addressing long-standing problems with the block layer's programming model. We continue to have IOThread bugs because there are many undocumented assumptions. With the multi-queue block layer the code switches to a more well-understood multi-threaded programming model and hopefully fewer issues will arise because there is no problematic AioContext lock to worry about. Stefan
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