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

From: Paolo Bonzini <pbonzini@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>,
	Emanuele Giuseppe Esposito <eesposit@redhat.com>
Cc: Fam Zheng <fam@euphon.net>,
	Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	qemu-block@nongnu.org, qemu-devel@nongnu.org,
	Hanna Reitz <hreitz@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	John Snow <jsnow@redhat.com>
Subject: Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept
Date: Wed, 13 Apr 2022 22:46:36 +0200	[thread overview]
Message-ID: <c8131010-ace5-b249-f3d1-0c0c86817439@redhat.com> (raw)
In-Reply-To: <Ylbjd3kzEsBZmgJQ@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