On Thu, Jul 15, 2021 at 03:35:37PM +0300, Vladimir Sementsov-Ogievskiy wrote: > 13.07.2021 19:38, Stefan Hajnoczi wrote: > > 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. > > > Hmm, do you mean that all jobs are thread-safe? > > Looking at the mirror, what protects s->ops_in_flight for example? It's > accessed from job coroutines and from mirror_top filter write operation.
You're right. I missed the bdrv_mirror_top BlockDriver: .pwrite_zeroes -> bdrv_mirror_top_pwrite_zeroes -> active_write_prepare -> QTAILQ_INSERT_TAIL(&s->ops_in_flight, op, next) This is not thread-safe. A CoMutex is needed here to protect MirrorBSDOpaque fields. Stefan
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