On 23/08/2017 16:51, Eric Blake wrote: > On 08/22/2017 07:51 AM, Stefan Hajnoczi wrote: >> The following scenario leads to an assertion failure in >> qio_channel_yield(): >> >> 1. Request coroutine calls qio_channel_yield() successfully when sending >> would block on the socket. It is now yielded. >> 2. nbd_read_reply_entry() calls nbd_recv_coroutines_enter_all() because >> nbd_receive_reply() failed. >> 3. Request coroutine is entered and returns from qio_channel_yield(). >> Note that the socket fd handler has not fired yet so >> ioc->write_coroutine is still set. >> 4. Request coroutine attempts to send the request body with nbd_rwv() >> but the socket would still block. qio_channel_yield() is called >> again and assert(!ioc->write_coroutine) is hit. >> >> The problem is that nbd_read_reply_entry() does not distinguish between >> request coroutines that are waiting to receive a reply and those that >> are not. >> >> This patch adds a per-request bool receiving flag so >> nbd_read_reply_entry() can avoid spurious aio_wake() calls. >> >> Reported-by: Dr. David Alan Gilbert <dgilb...@redhat.com> >> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> >> --- >> This should fix the issue that Dave is seeing but I'm concerned that >> there are more problems in nbd-client.c. We don't have good >> abstractions for writing coroutine socket I/O code. Something like Go's >> channels would avoid manual low-level coroutine calls. There is >> currently no way to cancel qio_channel_yield() so requests doing I/O may >> remain in-flight indefinitely and nbd-client.c doesn't join them... > > Vladimir has some cleanups that rewrite the NBD coroutines to be more > legible, but it is invasive enough to be 2.11 material. I think that > for a stop-gap of getting 2.10 out the door, we may be better off > including this patch - but I would still like some positive review from > more than just me. There's not much time left before I need to send the > -rc4 NBD pull request, though. >
Reviewed-by: Paolo Bonzini <pbonz...@redhat.com>
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