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

[Sergio Lopez]

On Tue, Sep 11, 2018 at 11:17:20AM +0200, Kevin Wolf wrote:
> 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.

Isn't this a consequence of using qemu_coroutine_enter in co_schedule_bh
[1]?

AFAIK, even if an IOThread's AioContext is being polled by the main loop
thread, all coroutines should be running with the IOThread/BDS
AioContext.

Sergio.

[1] https://lists.gnu.org/archive/html/qemu-devel/2018-09/msg00450.html