Re: [PATCH 8/8] graph-lock: Honour read locks even in the main thread

[Eric Blake]

On Wed, May 10, 2023 at 10:36:01PM +0200, Kevin Wolf wrote:
> 
> There are some conditions under which we don't actually need to do
> anything for taking a reader lock: Writing the graph is only possible
> from the main context while holding the BQL. So if a reader is running
> in the main context under the BQL and knows that it won't be interrupted
> until the next writer runs, we don't actually need to do anything.
> 
> This is the case if the reader code neither has a nested event loop
> (this is forbidden anyway while you hold the lock) nor is a coroutine
> (because a writer could run when the coroutine has yielded).
> 
> These conditions are exactly what bdrv_graph_rdlock_main_loop() asserts.
> They are not fulfilled in bdrv_graph_co_rdlock(), which always runs in a
> coroutine.
> 
> This deletes the shortcuts in bdrv_graph_co_rdlock() that skip taking
> the reader lock in the main thread.
> 
> Reported-by: Fiona Ebner <f.ebner@proxmox.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/graph-lock.c | 10 ----------
>  1 file changed, 10 deletions(-)

Bug fix by deletion is always fun.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
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