Re: [PATCH 7/8] qio: Use AioContext for default-context QIONetListener

[Kevin Wolf <kwolf@redhat.com>]

Am 03.11.2025 um 21:10 hat Eric Blake geschrieben:
> The user "John Doe" reported a deadlock when attempting to use
> qemu-storage-daemon to serve both a base file over NBD, and a qcow2
> file with that NBD export as its backing file, from the same process,
> even though it worked just fine when there were two q-s-d processes.
> The bulk of the NBD server code properly uses coroutines to make
> progress in an event-driven manner, but the code for spawning a new
> coroutine at the point when listen(2) detects a new client was
> hard-coded to use the global GMainContext; in other words, the
> callback that triggers nbd_client_new to let the server start the
> negotiation sequence with the client requires the main loop to be
> making progress.  However, the code for bdrv_open of a qcow2 image
> with an NBD backing file uses an AIO_WAIT_WHILE nested event loop to
> ensure that the entire qcow2 backing chain is either fully loaded or
> rejected, without any side effects from the main loop causing unwanted
> changes to the disk being loaded (in short, an AioContext represents
> the set of actions that are known to be safe while handling block
> layer I/O, while excluding any other pending actions in the global
> main loop with potentially larger risk of unwanted side effects).
> 
> This creates a classic case of deadlock: the server can't progress to
> the point of accept(2)ing the client to write to the NBD socket
> because the main loop is being starved until the AIO_WAIT_WHILE
> completes the bdrv_open, but the AIO_WAIT_WHILE can't progress because
> it is blocked on the client coroutine stuck in a read() of the
> expected magic number from the server side of the socket.
> 
> Fortunately, the way that AioContext is set up, any callback that is
> registered to the global AioContext will also be serviced by the main
> loop.  So the fix for the deadlock is to alter QIONetListener so that
> if it is not being used in an explicit alternative GMainContext, then
> it should perform its polling via the global AioContext (which
> indirectly still progresses in the default GMainContext) rather than
> directly in the default GMainContext.  This has no change in behavior
> to any prior use that did not starve the main loop, but has the
> additional benefit that in the bdrv_open case of a nested AioContext
> loop, the server's listen/accept handler is no longer starved because
> it is now part of the same AioContext loop.  From there, since NBD
> already uses coroutines for both server and client code, the nested
> AioContext loop finishes quickly and opening the qcow2 backing chain
> no longer deadlocks.
> 
> The next patch will add a unit test (kept separate to make it easier
> to rearrange the series to demonstrate the deadlock without this
> patch).
> 
> Fixes: https://gitlab.com/qemu-project/qemu/-/issues/3169
> Signed-off-by: Eric Blake <eblake@redhat.com>

This assumes that you'll only ever want to listen on a socket in the
main thread. I understand this is what the NBD server does today even if
you specify an iothread, so despite the limitation the patch is useful.
We just might need to change the interfaces again later.

Kevin