Re: [Qemu-devel] [PATCH 00/54] New op blocker system, part 1

[Kevin Wolf]

Am 21.02.2017 um 15:57 hat Kevin Wolf geschrieben:
> This series is the first part of implementing the new op blocker system
> whose design was agreed on quite a while ago, but proved a bit tricky to
> implement in places. There is more work to do to fully replace the old op
> blocker system, but realistically we don't have that much time until the 2.9
> freeze. So let's merge this series to complement the traditional op blockers
> and plan with a second part for the 2.10 timeframe.
> 
> The basic idea is that every user of a block node (including things outside 
> the
> block layer that go through a BlockBackend, and also other block nodes that
> hold references to it) has to declare which low-level operations/permissions 
> it
> needs and which operation it allows other users to perform on the same node.
> Depending on these declarations, conflicts are avoided by returning an error
> for attempts to attach a conflicting user to the same node.
> 
> After this series, all users request permissions, and hopefully all of the
> permissions they need. For writes and resize, getting the permission first is
> actually enforced with assertions. Asserting it for consistent reads is in
> theory doable, but would mean introducing a request flag that tell us that
> inconsistent reads are okay - and that in all block drivers to recursively
> propagate this flag through the backing chain. It might not be worth it.
> 
> As stated above, the series doesn't remove the old op blockers yet, though in
> theory the new op blockers should block everything that needs to be blocked.
> In practice, the read/write/resize blockers should be okay, but
> BLK_PERM_GRAPH_MOD isn't to be taken too seriously at the moment. It isn't
> really applied consistently and doesn't do much useful yet. Making proper use
> of it is left for the part 2 series.

In order to keep the pull request next week at least a bit smaller, I
applied the already reviewed patches 1-11 to the block branch (i.e.
the preparations before the actual op blocker patches start).

Kevin