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Subject: Re: [PATCH 2/2] nbd/server: Quiesce coroutines on context switch Message-ID: <2a1a0d48-5c0c-fa6a-a648-c65fc8140156@redhat.com> Date: Fri, 4 Dec 2020 12:39:07 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <20201204165347.73542-3-slp@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eblake@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.496, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Stefan Hajnoczi , qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 12/4/20 10:53 AM, Sergio Lopez wrote: > When switching between AIO contexts we need to me make sure that both > recv_coroutine and send_coroutine are not scheduled to run. Otherwise, > QEMU may crash while attaching the new context with an error like > this one: > > aio_co_schedule: Co-routine was already scheduled in 'aio_co_schedule' > > To achieve this we need a local implementation of > 'qio_channel_readv_all_eof' named 'nbd_read_eof' (a trick already done > by 'nbd/client.c') that allows us to interrupt the operation and to > know when recv_coroutine is yielding. > > With this in place, we delegate detaching the AIO context to the > owning context with a BH ('nbd_aio_detach_bh') scheduled using > 'aio_wait_bh_oneshot'. This BH signals that we need to quiesce the > channel by setting 'client->quiescing' to 'true', and either waits for > the coroutine to finish using AIO_WAIT_WHILE or, if it's yielding in > 'nbd_read_eof', actively enters the coroutine to interrupt it. > > RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1900326 > Signed-off-by: Sergio Lopez > --- > nbd/server.c | 120 +++++++++++++++++++++++++++++++++++++++++++++------ > 1 file changed, 106 insertions(+), 14 deletions(-) A complex patch, so I'd appreciate a second set of eyes. > > diff --git a/nbd/server.c b/nbd/server.c > index 613ed2634a..7229f487d2 100644 > --- a/nbd/server.c > +++ b/nbd/server.c > @@ -132,6 +132,9 @@ struct NBDClient { > CoMutex send_lock; > Coroutine *send_coroutine; > > + bool read_yielding; > + bool quiescing; Will either of these fields need to be accessed atomically once the 'yank' code is added, or are we still safe with direct access because coroutines are not multithreaded? > + > QTAILQ_ENTRY(NBDClient) next; > int nb_requests; > bool closing; > @@ -1352,14 +1355,60 @@ static coroutine_fn int nbd_negotiate(NBDClient *client, Error **errp) > return 0; > } > > -static int nbd_receive_request(QIOChannel *ioc, NBDRequest *request, > +/* nbd_read_eof > + * Tries to read @size bytes from @ioc. This is a local implementation of > + * qio_channel_readv_all_eof. We have it here because we need it to be > + * interruptible and to know when the coroutine is yielding. > + * Returns 1 on success > + * 0 on eof, when no data was read (errp is not set) > + * negative errno on failure (errp is set) > + */ > +static inline int coroutine_fn > +nbd_read_eof(NBDClient *client, void *buffer, size_t size, Error **errp) > +{ > + bool partial = false; > + > + assert(size); > + while (size > 0) { > + struct iovec iov = { .iov_base = buffer, .iov_len = size }; > + ssize_t len; > + > + len = qio_channel_readv(client->ioc, &iov, 1, errp); > + if (len == QIO_CHANNEL_ERR_BLOCK) { > + client->read_yielding = true; > + qio_channel_yield(client->ioc, G_IO_IN); > + client->read_yielding = false; nbd/client.c:nbd_read_eof() uses bdrv_dec/inc_in_flight instead of read_yielding... > + if (client->quiescing) { > + return -EAGAIN; > + } and the quiescing check is new; otherwise, these two functions look identical. Having two static functions with the same name makes gdb a bit more annoying (which one of the two did you want your breakpoint on?). Is there any way we could write this code only once in nbd/common.c for reuse by both client and server? But I can live with it as written. > @@ -2151,20 +2223,23 @@ static int nbd_co_send_bitmap(NBDClient *client, uint64_t handle, > > /* nbd_co_receive_request > * Collect a client request. Return 0 if request looks valid, -EIO to drop > - * connection right away, and any other negative value to report an error to > - * the client (although the caller may still need to disconnect after reporting > - * the error). > + * connection right away, -EAGAIN to indicate we were interrupted and the > + * channel should be quiesced, and any other negative value to report an error > + * to the client (although the caller may still need to disconnect after > + * reporting the error). > */ > static int nbd_co_receive_request(NBDRequestData *req, NBDRequest *request, > Error **errp) > { > NBDClient *client = req->client; > int valid_flags; > + int ret; > > g_assert(qemu_in_coroutine()); > assert(client->recv_coroutine == qemu_coroutine_self()); > - if (nbd_receive_request(client->ioc, request, errp) < 0) { > - return -EIO; > + ret = nbd_receive_request(client, request, errp); > + if (ret < 0) { > + return ret; Why the double space? The old code slams to EIO, you preserve errors. Is that going to bite us by causing us to see a different errno leaked through? > } > > trace_nbd_co_receive_request_decode_type(request->handle, request->type, > @@ -2507,6 +2582,17 @@ static coroutine_fn void nbd_trip(void *opaque) > return; > } > > + if (client->quiescing) { > + /* > + * We're switching between AIO contexts. Don't attempt to receive a new > + * request and kick the main context which may be waiting for us. s/request/request,/ > + */ > + nbd_client_put(client); > + client->recv_coroutine = NULL; > + aio_wait_kick(); > + return; > + } > + > req = nbd_request_get(client); > ret = nbd_co_receive_request(req, &request, &local_err); > client->recv_coroutine = NULL; > @@ -2519,6 +2605,11 @@ static coroutine_fn void nbd_trip(void *opaque) > goto done; > } > > + if (ret == -EAGAIN) { > + assert(client->quiescing); > + goto done; > + } > + > nbd_client_receive_next_request(client); > if (ret == -EIO) { > goto disconnect; > @@ -2565,7 +2656,8 @@ disconnect: > > static void nbd_client_receive_next_request(NBDClient *client) > { > - if (!client->recv_coroutine && client->nb_requests < MAX_NBD_REQUESTS) { > + if (!client->recv_coroutine && client->nb_requests < MAX_NBD_REQUESTS && > + !client->quiescing) { > nbd_client_get(client); > client->recv_coroutine = qemu_coroutine_create(nbd_trip, client); > aio_co_schedule(client->exp->common.ctx, client->recv_coroutine); > Overall looks okay to me, Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org