Re: [PATCH v4 for-6.0? 0/3] qcow2: fix parallel rewrite and discard (rw-lock)

[Max Reitz]

On 25.03.21 20:12, Vladimir Sementsov-Ogievskiy wrote:
> ping. Do we want it for 6.0?

I’d rather wait.  I think the conclusion was that guests shouldn’t hit 
this because they serialize discards?

There’s also something Kevin wrote on IRC a couple of weeks ago, for 
which I had hoped he’d sent an email but I don’t think he did, so I’ll 
try to remember and paraphrase as well as I can...

He basically asked whether it wouldn’t be conceptually simpler to take a 
reference to some cluster in get_cluster_offset() and later release it 
with a to-be-added put_cluster_offset().

He also noted that reading is problematic, too, because if you read a 
discarded and reused cluster, this might result in an information leak 
(some guest application might be able to read data it isn’t allowed to 
read); that’s why making get_cluster_offset() the point of locking 
clusters against discarding would be better.

This would probably work with both of your solutions.  For the in-memory 
solutions, you’d take a refcount to an actual cluster; in the CoRwLock 
solution, you’d take that lock.

What do you think?

Max