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

[Paolo Bonzini]

On 4/13/22 18:29, Kevin Wolf wrote:
> > A reader does not have to be a coroutine. AIO_WAIT_WHILE is not
> > mandatory to allow it to finish, it helps to ensure progress in case
> > some reader is waiting for something, but other than that is not
> > necessary IMO.
> When it's outside of a coroutine, how would you implement waiting for a
> writer to finish if not with AIO_WAIT_WHILE()?

In the main thread a non-coroutine can always read the graph though, 
because the only writer can be the main thread.

If the critical sections are large enough, I don't think rdlock needs to 
be taken outside a coroutine in the iothread, e.g. in a bottom half.

> > No I think if we focus on small pieces of code we end up having a
> > million lock/unlock pairs.
>
> Yes, I agree. On the other hand, if we're taking the locks in high-level
> outer operations, avoiding to take the lock recursively might become
> harder. I guess we'll see how it works out when we actually introduce
> callers.

My "hope" is that taking the locks in blk_* functions covers most of the 
calls, and then only a few (dozens) direct uses of bdrv_* remain.

Unfortunately, adding assertions is not easy because "is there a reader" 
cannot be easily answered.  But I think Emanuele has a debug mode that 
can enable the assertions at a performance cost.

Paolo