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

[Kevin Wolf <kwolf@redhat.com>]

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.

Kevin