Re: [Qemu-devel] [PATCH 06/14] block: Add missing locking in bdrv_co_drain_bh_cb()

[Kevin Wolf <kwolf@redhat.com>]

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 <kwolf@redhat.com>
> > ---
> >  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();