[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 06/14] block: Add missing locking in bdrv_co_dra
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH 06/14] block: Add missing locking in bdrv_co_drain_bh_cb() |
Date: |
Tue, 11 Sep 2018 11:17:20 +0200 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
Am 11.09.2018 um 10:23 hat Fam Zheng geschrieben:
> On Fri, 09/07 18:15, Kevin Wolf wrote:
> > bdrv_do_drained_begin/end() assume that they are called with the
> > AioContext lock of bs held. If we call drain functions from a coroutine
> > with the AioContext lock held, we yield and schedule a BH to move out of
> > coroutine context. This means that the lock for the home context of the
> > coroutine is released and must be re-acquired in the bottom half.
> >
> > Signed-off-by: Kevin Wolf <address@hidden>
> > ---
> > include/qemu/coroutine.h | 5 +++++
> > block/io.c | 15 +++++++++++++++
> > util/qemu-coroutine.c | 5 +++++
> > 3 files changed, 25 insertions(+)
> >
> > diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
> > index 6f8a487041..9801e7f5a4 100644
> > --- a/include/qemu/coroutine.h
> > +++ b/include/qemu/coroutine.h
> > @@ -90,6 +90,11 @@ void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine
> > *co);
> > void coroutine_fn qemu_coroutine_yield(void);
> >
> > /**
> > + * Get the AioContext of the given coroutine
> > + */
> > +AioContext *coroutine_fn qemu_coroutine_get_aio_context(Coroutine *co);
> > +
> > +/**
> > * Get the currently executing coroutine
> > */
> > Coroutine *coroutine_fn qemu_coroutine_self(void);
> > diff --git a/block/io.c b/block/io.c
> > index 7100344c7b..914ba78f1a 100644
> > --- a/block/io.c
> > +++ b/block/io.c
> > @@ -288,6 +288,18 @@ static void bdrv_co_drain_bh_cb(void *opaque)
> > BlockDriverState *bs = data->bs;
> >
> > if (bs) {
> > + AioContext *ctx = bdrv_get_aio_context(bs);
> > + AioContext *co_ctx = qemu_coroutine_get_aio_context(co);
> > +
> > + /*
> > + * When the coroutine yielded, the lock for its home context was
> > + * released, so we need to re-acquire it here. If it explicitly
> > + * acquired a different context, the lock is still held and we
> > don't
> > + * want to lock it a second time (or AIO_WAIT_WHILE() would hang).
> > + */
>
> This condition is rather obscure. When is ctx not equal to co_ctx?
Whenever you drain a BlockDriverState that is in a different AioContext.
The common case is a bdrv_drain() from the main loop thread for a BDS in
an iothread.
I didn't have this condition at first and ran into deadlocks (because
AIO_WAIT_WHILE() dropped the lock only once, but it was locked twice).
Kevin
> > + if (ctx == co_ctx) {
> > + aio_context_acquire(ctx);
> > + }
> > bdrv_dec_in_flight(bs);
> > if (data->begin) {
> > bdrv_do_drained_begin(bs, data->recursive, data->parent,
> > @@ -296,6 +308,9 @@ static void bdrv_co_drain_bh_cb(void *opaque)
> > bdrv_do_drained_end(bs, data->recursive, data->parent,
> > data->ignore_bds_parents);
> > }
> > + if (ctx == co_ctx) {
> > + aio_context_release(ctx);
> > + }
> > } else {
> > assert(data->begin);
> > bdrv_drain_all_begin();
- [Qemu-devel] [PATCH 04/14] job: Use AIO_WAIT_WHILE() in job_finish_sync(), (continued)
- [Qemu-devel] [PATCH 04/14] job: Use AIO_WAIT_WHILE() in job_finish_sync(), Kevin Wolf, 2018/09/07
- [Qemu-devel] [PATCH 07/14] aio-wait: Increase num_waiters even in home thread, Kevin Wolf, 2018/09/07
- [Qemu-devel] [PATCH 03/14] test-blockjob: Acquire AioContext around job_finish_sync(), Kevin Wolf, 2018/09/07
- [Qemu-devel] [PATCH 05/14] test-bdrv-drain: Test AIO_WAIT_WHILE() in completion callback, Kevin Wolf, 2018/09/07
- [Qemu-devel] [PATCH 06/14] block: Add missing locking in bdrv_co_drain_bh_cb(), Kevin Wolf, 2018/09/07
[Qemu-devel] [PATCH 11/14] mirror: Fix potential use-after-free in active commit, Kevin Wolf, 2018/09/07
[Qemu-devel] [PATCH 08/14] block-backend: Add .drained_poll callback, Kevin Wolf, 2018/09/07
[Qemu-devel] [PATCH 10/14] block-backend: Decrease in_flight only after callback, Kevin Wolf, 2018/09/07
[Qemu-devel] [PATCH 13/14] block: Remove aio_poll() in bdrv_drain_poll variants, Kevin Wolf, 2018/09/07