Re: [PATCH 11/13] block/blkdebug: make a breakpoint work across AioContexts

[Eric Blake <eblake@redhat.com>]

On Wed, Sep 30, 2026 at 06:49:57PM +0200, Denis V. Lunev wrote:
> From: Denis V. Lunev <den@openvz.org>
> 
> A request that stops at a breakpoint belongs to whichever AioContext
> issued it, and the monitor or qemu-io that resumes it runs somewhere
> else. Two things go wrong once those are not the same thread.
> 
> resume_req_by_tag() woke the request with qemu_coroutine_enter(), which
> enters the coroutine on the caller's thread. That moves the request off
> its own thread for the rest of its life, and it aborts outright when it
> wins the race against the yield, because the request is published to
> the list before the coroutine gets there.
> 
> wait_break polled the block backend's AioContext, where the event will
> never arrive, so it slept through the suspension it was waiting for.
> 
> Wake the request in the context it came from and wait for it the way
> the rest of the block layer waits across contexts. Where the two
> contexts are the same the coroutine is still entered directly, so
> single-threaded behaviour, and the output tests depending on it, do not
> change.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Hanna Reitz <hreitz@redhat.com>
> CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> ---
>  block/blkdebug.c | 3 ++-
>  qemu-io-cmds.c   | 5 ++---
>  2 files changed, 4 insertions(+), 4 deletions(-)

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org