[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [RFC PATCH 4/5] child_job_drained_poll: override polling condition o
From: |
Stefan Hajnoczi |
Subject: |
Re: [RFC PATCH 4/5] child_job_drained_poll: override polling condition only when in home thread |
Date: |
Wed, 2 Mar 2022 16:37:18 +0000 |
On Tue, Mar 01, 2022 at 09:21:12AM -0500, Emanuele Giuseppe Esposito wrote:
> drv->drained_poll() is only implemented in mirror, and allows
> it to drain from the coroutine. The mirror implementation uses
> in_drain flag to recognize when it is draining from coroutine,
> and consequently avoid deadlocking (wait the poll condition in
> child_job_drained_poll to wait for itself).
>
> The problem is that this flag is dangerous, because it breaks
> bdrv_drained_begin() invariants: once drained_begin ends, all
> jobs, in_flight requests, and anything running in the iothread
> are blocked.
>
> This can be broken in such way:
> iothread(mirror): s->in_drain = true; // mirror.c:1112
> main loop: bdrv_drained_begin(mirror_bs);
> /*
> * drained_begin wait for bdrv_drain_poll_top_level() condition,
> * that translates in child_job_drained_poll() for jobs, but
> * mirror implements drv->drained_poll() so it returns
> * !!in_flight_requests, which his 0 (assertion in mirror.c:1105).
> */
> main loop: thinks iothread is stopped and is modifying the graph...
> iothread(mirror): *continues*, as nothing is stopping it
> iothread(mirror): bdrv_drained_begin(bs);
> /* draining reads the graph while it is modified!! */
> main loop: done modifying the graph...
>
> In order to fix this, we can simply allow drv->drained_poll()
> to be called only by the iothread, and not the main loop.
> We distinguish it by using in_aio_context_home_thread(), that
> returns false if @ctx is not the same as the thread that runs it.
>
> Co-Developed-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
> blockjob.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/blockjob.c b/blockjob.c
> index 10815a89fe..e132d9587e 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -107,6 +107,7 @@ static bool child_job_drained_poll(BdrvChild *c)
> BlockJob *bjob = c->opaque;
> Job *job = &bjob->job;
> const BlockJobDriver *drv = block_job_driver(bjob);
> + AioContext *ctx = block_job_get_aio_context(bjob);
>
> /* An inactive or completed job doesn't have any pending requests. Jobs
> * with !job->busy are either already paused or have a pause point after
> @@ -117,7 +118,7 @@ static bool child_job_drained_poll(BdrvChild *c)
>
> /* Otherwise, assume that it isn't fully stopped yet, but allow the job
> to
> * override this assumption. */
> - if (drv->drained_poll) {
> + if (in_aio_context_home_thread(ctx) && drv->drained_poll) {
> return drv->drained_poll(bjob);
The purpose of in_aio_context_home_thread() is not obvious from the
code. Please add a comment explaining this case.
Thanks,
Stefan
signature.asc
Description: PGP signature
- [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept, Emanuele Giuseppe Esposito, 2022/03/01
- [RFC PATCH 2/5] introduce BDRV_POLL_WHILE_UNLOCKED, Emanuele Giuseppe Esposito, 2022/03/01
- [RFC PATCH 4/5] child_job_drained_poll: override polling condition only when in home thread, Emanuele Giuseppe Esposito, 2022/03/01
- Re: [RFC PATCH 4/5] child_job_drained_poll: override polling condition only when in home thread,
Stefan Hajnoczi <=
- [RFC PATCH 5/5] test-bdrv-drain: ensure draining from main loop stops iothreads, Emanuele Giuseppe Esposito, 2022/03/01
- [RFC PATCH 3/5] block/io.c: introduce bdrv_subtree_drained_{begin/end}_unlocked, Emanuele Giuseppe Esposito, 2022/03/01
- [RFC PATCH 1/5] aio-wait.h: introduce AIO_WAIT_WHILE_UNLOCKED, Emanuele Giuseppe Esposito, 2022/03/01
- Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept, Emanuele Giuseppe Esposito, 2022/03/01
- Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept, Stefan Hajnoczi, 2022/03/02