Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept

[Paolo Bonzini]

On 3/30/22 18:02, Paolo Bonzini wrote:
> On 3/30/22 12:53, Hanna Reitz wrote:
> > > Seems a good compromise between drains and rwlock. What do you think?
> >
> > Well, sounds complicated.  So I’m asking myself whether this would be 
> > noticeably better than just an RwLock for graph modifications, like 
> > the global lock Vladimir has proposed.

[try again, this time with brain connected]

A global lock would have to be taken by all iothreads on every I/O
operation, even a CoRwLock would not scale because it has a global
CoMutex inside and rdlock/unlock both take it.  Even if the critical
section is small, the cacheline bumping would be pretty bad.

Perhaps we could reuse the code in cpus-common.c, which relies on a list
of possible readers and is quite cheap (two memory barriers basically)
for readers.  Here we would have a list of AioContexts (or perhaps 
BlockDriverStates?) as the possible readers.

The slow path uses a QemuMutex, a QemuCond for the readers and a
QemuCond for the writers.  The reader QemuCond can be replaced by a
CoQueue, I think.

Paolo