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Re: [Qemu-block] [RFC PATCH 4/5] block: Drop AioContext lock in bdrv_dra
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-block] [RFC PATCH 4/5] block: Drop AioContext lock in bdrv_drain_poll_top_level() |
Date: |
Tue, 4 Sep 2018 16:50:08 +0200 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
Am 24.08.2018 um 09:24 hat Fam Zheng geschrieben:
> On Fri, 08/17 19:02, Kevin Wolf wrote:
> > Simimlar to AIO_WAIT_WHILE(), bdrv_drain_poll_top_level() needs to
> > release the AioContext lock of the node to be drained before calling
> > aio_poll(). Otherwise, callbacks called by aio_poll() would possibly
> > take the lock a second time and run into a deadlock with a nested
> > AIO_WAIT_WHILE() call.
> >
> > Signed-off-by: Kevin Wolf <address@hidden>
> > ---
> > block/io.c | 25 ++++++++++++++++++++++++-
> > 1 file changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/block/io.c b/block/io.c
> > index 7100344c7b..832d2536bf 100644
> > --- a/block/io.c
> > +++ b/block/io.c
> > @@ -268,9 +268,32 @@ bool bdrv_drain_poll(BlockDriverState *bs, bool
> > recursive,
> > static bool bdrv_drain_poll_top_level(BlockDriverState *bs, bool recursive,
> > BdrvChild *ignore_parent)
> > {
> > + AioContext *ctx = bdrv_get_aio_context(bs);
> > +
> > + /*
> > + * We cannot easily release the lock unconditionally here because many
> > + * callers of drain function (like qemu initialisation, tools, etc.)
> > don't
> > + * even hold the main context lock.
> > + *
> > + * This means that we fix potential deadlocks for the case where we
> > are in
> > + * the main context and polling a BDS in a different AioContext, but
> > + * draining a BDS in the main context from a different I/O thread would
> > + * still have this problem. Fortunately, this isn't supposed to happen
> > + * anyway.
> > + */
> > + if (ctx != qemu_get_aio_context()) {
> > + aio_context_release(ctx);
> > + } else {
> > + assert(qemu_get_current_aio_context() == qemu_get_aio_context());
> > + }
> > +
> > /* Execute pending BHs first and check everything else only after the
> > BHs
> > * have executed. */
> > - while (aio_poll(bs->aio_context, false));
> > + while (aio_poll(ctx, false));
> > +
> > + if (ctx != qemu_get_aio_context()) {
> > + aio_context_acquire(ctx);
> > + }
> >
> > return bdrv_drain_poll(bs, recursive, ignore_parent, false);
> > }
>
> The same question as patch 3: why not just use AIO_WAIT_WHILE() here? It takes
> care to not release any lock if both running and polling in the main context
> (taking the in_aio_context_home_thread() branch).
I don't think AIO_WAIT_WHILE() can be non-blocking, though?
There is also no real condition here to check. It's just polling as long
as there is activity to get the pending BH callbacks run. I'm not sure
how I could possibly write this as a AIO_WAIT_WHILE() condition.
After all, this one just doesn't feel like the right use case for
AIO_WAIT_WHILE().
Kevin
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