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

[Paolo Bonzini <pbonzini@redhat.com>]

On 4/13/22 16:51, Kevin Wolf wrote:
> So the idea is that we can do bdrv_graph_co_rdlock() in one thread and
> the corresponding bdrv_graph_co_rdunlock() in a different thread?
> 
> Would the unlock somehow remember the original thread, or do you use the
> "sum is correct" argument and allow negative counter values, so you can
> end up having count +1 in A and -1 in B to represent "no active
> readers"? If this happens, it's likely to happen many times, so do we
> have to take integer overflows into account then?

The counter cannot be negative, so you can use uint32_t and sum modulo 
2^32.  You might have a thread with counter 2^31+1 which is negative in 
twos complement; and a thread with counter -2^31-1 which is positive in 
twos complement; but their sum cancels out correctly.

Paolo