Re: [RFC PATCH 2/3] block: Allow bdrv_run_co() from different AioContext

[Kevin Wolf <kwolf@redhat.com>]

Am 25.05.2020 um 18:41 hat Kevin Wolf geschrieben:
> Am 25.05.2020 um 16:18 hat Stefan Reiter geschrieben:
> > On 5/12/20 4:43 PM, Kevin Wolf wrote:
> > > Coroutine functions that are entered through bdrv_run_co() are already
> > > safe to call from synchronous code in a different AioContext because
> > > bdrv_coroutine_enter() will schedule them in the context of the node.
> > > 
> > > However, the coroutine fastpath still requires that we're already in the
> > > right AioContext when called in coroutine context.
> > > 
> > > In order to make the behaviour more consistent and to make life a bit
> > > easier for callers, let's check the AioContext and automatically move
> > > the current coroutine around if we're not in the right context yet.
> > > 
> > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > > ---
> > >   block/io.c | 15 ++++++++++++++-
> > >   1 file changed, 14 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/block/io.c b/block/io.c
> > > index c1badaadc9..7808e8bdc0 100644
> > > --- a/block/io.c
> > > +++ b/block/io.c
> > > @@ -895,8 +895,21 @@ static int bdrv_run_co(BlockDriverState *bs, CoroutineEntry *entry,
> > >                          void *opaque, int *ret)
> > >   {
> > >       if (qemu_in_coroutine()) {
> > > -        /* Fast-path if already in coroutine context */
> > > +        Coroutine *self = qemu_coroutine_self();
> > > +        AioContext *bs_ctx = bdrv_get_aio_context(bs);
> > > +        AioContext *co_ctx = qemu_coroutine_get_aio_context(self);
> > > +
> > > +        if (bs_ctx != co_ctx) {
> > > +            /* Move to the iothread of the node */
> > > +            aio_co_schedule(bs_ctx, self);
> > > +            qemu_coroutine_yield();
> > 
> > I'm pretty sure this can lead to a race: When the thread we're re-scheduling
> > to is faster to schedule us than we can reach qemu_coroutine_yield, then
> > we'll get an abort ("Co-routine re-entered recursively"), since co->caller
> > is still set.
> > 
> > I've seen this happen in our code when I try to do the scheduling fandangle
> > there.
> 
> Ah, crap. I guess letting a coroutine re-schedule itself is only safe
> within the same thread then.
> 
> > Is there a safer way to have a coroutine reschedule itself? Some lock
> > missing?
> 
> There is no problem that can't be solved by adding another level of
> indirection... We would have to schedule a BH in the original thread
> that will only schedule the coroutine in its new thread after it has
> yielded.
> 
> Maybe we should actually introduce a helper function that moves the
> current coroutine to a different AioContext this way.

Like this:

https://repo.or.cz/qemu/kevin.git/commitdiff/ed0244ba4ac699f7e8eaf7512ff25645cf43bda2

The series for which I need this isn't quite ready yet, so I haven't
sent it as a patch yet, but if it proves useful in other contexts, we
can always commit it without the rest.

Kevin