| Message ID | 20210125180115.22936-5-vgoyal@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | vhost-user: Shutdown/Flush slave channel properly | expand |
On Mon, Jan 25, 2021 at 01:01:13PM -0500, Vivek Goyal wrote: > Currently we don't have a mechanism to flush slave channel while shutting > down vhost-user device and that can result a deadlock. Consider following > scenario. > > 1. Slave gets a request from guest on virtqueue (say index 1, vq1), to map > a portion of file in qemu address space. > > 2. Thread serving vq1 receives this request and sends a message to qemu on > slave channel/fd and gets blocked in waiting for a response from qemu. > > 3. In the mean time, user does "echo b > /proc/sysrq-trigger" in guest. This > leads to qemu reset and ultimately in main thread we end up in > vhost_dev_stop() which is trying to shutdown virtqueues for the device. > > 4. Slave gets VHOST_USER_GET_VRING_BASE message to shutdown a virtqueue on > unix socket being used for communication. > > 5. Slave tries to shutdown the thread serving vq1 and waits for it to > terminate. But vq1 thread can't terminate because it is waiting for > a response from qemu on slave_fd. And qemu is not processing slave_fd > anymore as qemu is ressing (and not running main event loop anymore) > and is waiting for a response to VHOST_USER_GET_VRING_BASE. > > So we have a deadlock. Qemu is waiting on slave to response to > VHOST_USER_GET_VRING_BASE message and slave is waiting on qemu to > respond to request it sent on slave_fd. > > I can reliably reproduce this race with virtio-fs. > > Hence here is the proposal to solve this problem. Enhance vhost-user > protocol to properly shutdown slave_fd channel. And if there are pending > requests, flush the channel completely before sending the request to > shutdown virtqueues. > > New workflow to shutdown slave channel is. > > - Qemu sends VHOST_USER_STOP_SLAVE_CHANNEL request to slave. It waits > for an reply from guest. > > - Then qemu sits in a tight loop waiting for > VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message from slave on slave_fd. > And while waiting for this message, qemu continues to process requests > on slave_fd to flush any pending requests. This will unblock threads > in slave and allow slave to shutdown slave channel. > > - Once qemu gets VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message, it knows > no more requests will come on slave_fd and it can continue to shutdown > virtqueues now. > > - Once device starts again, qemu will send VHOST_USER_START_SLAVE_CHANNEL > message to slave to open the slave channel and receive requests. > > IOW, this allows for proper shutdown of slave channel, making sure > no threads in slave are blocked on sending/receiving message. And > this in-turn allows for shutting down of virtqueues, hence resolving > the deadlock. Is the new message necessary? How about letting QEMU handle slave fd activity while waiting for virtqueues to stop instead? In other words, QEMU should monitor both the UNIX domain socket and the slave fd after it has sent VHOST_USER_GET_VRING_BASE and is awaiting the response. Stefan
On Thu, Jan 28, 2021 at 04:52:34PM +0000, Stefan Hajnoczi wrote: > On Mon, Jan 25, 2021 at 01:01:13PM -0500, Vivek Goyal wrote: > > Currently we don't have a mechanism to flush slave channel while shutting > > down vhost-user device and that can result a deadlock. Consider following > > scenario. > > > > 1. Slave gets a request from guest on virtqueue (say index 1, vq1), to map > > a portion of file in qemu address space. > > > > 2. Thread serving vq1 receives this request and sends a message to qemu on > > slave channel/fd and gets blocked in waiting for a response from qemu. > > > > 3. In the mean time, user does "echo b > /proc/sysrq-trigger" in guest. This > > leads to qemu reset and ultimately in main thread we end up in > > vhost_dev_stop() which is trying to shutdown virtqueues for the device. > > > > 4. Slave gets VHOST_USER_GET_VRING_BASE message to shutdown a virtqueue on > > unix socket being used for communication. > > > > 5. Slave tries to shutdown the thread serving vq1 and waits for it to > > terminate. But vq1 thread can't terminate because it is waiting for > > a response from qemu on slave_fd. And qemu is not processing slave_fd > > anymore as qemu is ressing (and not running main event loop anymore) > > and is waiting for a response to VHOST_USER_GET_VRING_BASE. > > > > So we have a deadlock. Qemu is waiting on slave to response to > > VHOST_USER_GET_VRING_BASE message and slave is waiting on qemu to > > respond to request it sent on slave_fd. > > > > I can reliably reproduce this race with virtio-fs. > > > > Hence here is the proposal to solve this problem. Enhance vhost-user > > protocol to properly shutdown slave_fd channel. And if there are pending > > requests, flush the channel completely before sending the request to > > shutdown virtqueues. > > > > New workflow to shutdown slave channel is. > > > > - Qemu sends VHOST_USER_STOP_SLAVE_CHANNEL request to slave. It waits > > for an reply from guest. > > > > - Then qemu sits in a tight loop waiting for > > VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message from slave on slave_fd. > > And while waiting for this message, qemu continues to process requests > > on slave_fd to flush any pending requests. This will unblock threads > > in slave and allow slave to shutdown slave channel. > > > > - Once qemu gets VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message, it knows > > no more requests will come on slave_fd and it can continue to shutdown > > virtqueues now. > > > > - Once device starts again, qemu will send VHOST_USER_START_SLAVE_CHANNEL > > message to slave to open the slave channel and receive requests. > > > > IOW, this allows for proper shutdown of slave channel, making sure > > no threads in slave are blocked on sending/receiving message. And > > this in-turn allows for shutting down of virtqueues, hence resolving > > the deadlock. > > Is the new message necessary? Hi Stefan, It probably is not necessary but it feels like a cleaner and simpler solution. There slave is a separate channel from virtqueues. And if device is stopping, we want to make sure there are no pending requests in slave channel. It is possible that virtqueue shutodwn is successful but other entity could still be sending messages on slave channel. In that case, shall we say device shutdown complete and stop polling slave channel? IOW, the way we have explicit protocol messages to shutdown each vring, it makes sense to have explicit protocol message to shutdown slave channel as well. Right now there is no mechanism to do that. IMHO, introducing an explicit mechanism in protocol to shutdown slave channel feels better as compared to say keep on serving slave channel in parallel while waiting for virtuqueues to shutdown and stop serving slave as soon as last virtuqueue is stopped (there could still be pending slave message right after last virtuqueue got shutdown). > How about letting QEMU handle slave fd > activity while waiting for virtqueues to stop instead? > > In other words, QEMU should monitor both the UNIX domain socket and the > slave fd after it has sent VHOST_USER_GET_VRING_BASE and is awaiting the > response. I can give it a try. If there is a strong preference for this solution. I guess I can patch vhost_user_get_vring_base() to poll and serve both unix domain socket and slave fd. But at this point of time I do think that adding a mechanism to shutodwn slave channel (the way we have mechanism to shutdown vring), sounds better from design point of view. Vivek
On Fri, Jan 29, 2021 at 09:16:00AM -0500, Vivek Goyal wrote: > On Thu, Jan 28, 2021 at 04:52:34PM +0000, Stefan Hajnoczi wrote: > > On Mon, Jan 25, 2021 at 01:01:13PM -0500, Vivek Goyal wrote: > > > Currently we don't have a mechanism to flush slave channel while shutting > > > down vhost-user device and that can result a deadlock. Consider following > > > scenario. > > > > > > 1. Slave gets a request from guest on virtqueue (say index 1, vq1), to map > > > a portion of file in qemu address space. > > > > > > 2. Thread serving vq1 receives this request and sends a message to qemu on > > > slave channel/fd and gets blocked in waiting for a response from qemu. > > > > > > 3. In the mean time, user does "echo b > /proc/sysrq-trigger" in guest. This > > > leads to qemu reset and ultimately in main thread we end up in > > > vhost_dev_stop() which is trying to shutdown virtqueues for the device. > > > > > > 4. Slave gets VHOST_USER_GET_VRING_BASE message to shutdown a virtqueue on > > > unix socket being used for communication. > > > > > > 5. Slave tries to shutdown the thread serving vq1 and waits for it to > > > terminate. But vq1 thread can't terminate because it is waiting for > > > a response from qemu on slave_fd. And qemu is not processing slave_fd > > > anymore as qemu is ressing (and not running main event loop anymore) > > > and is waiting for a response to VHOST_USER_GET_VRING_BASE. > > > > > > So we have a deadlock. Qemu is waiting on slave to response to > > > VHOST_USER_GET_VRING_BASE message and slave is waiting on qemu to > > > respond to request it sent on slave_fd. > > > > > > I can reliably reproduce this race with virtio-fs. > > > > > > Hence here is the proposal to solve this problem. Enhance vhost-user > > > protocol to properly shutdown slave_fd channel. And if there are pending > > > requests, flush the channel completely before sending the request to > > > shutdown virtqueues. > > > > > > New workflow to shutdown slave channel is. > > > > > > - Qemu sends VHOST_USER_STOP_SLAVE_CHANNEL request to slave. It waits > > > for an reply from guest. > > > > > > - Then qemu sits in a tight loop waiting for > > > VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message from slave on slave_fd. > > > And while waiting for this message, qemu continues to process requests > > > on slave_fd to flush any pending requests. This will unblock threads > > > in slave and allow slave to shutdown slave channel. > > > > > > - Once qemu gets VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message, it knows > > > no more requests will come on slave_fd and it can continue to shutdown > > > virtqueues now. > > > > > > - Once device starts again, qemu will send VHOST_USER_START_SLAVE_CHANNEL > > > message to slave to open the slave channel and receive requests. > > > > > > IOW, this allows for proper shutdown of slave channel, making sure > > > no threads in slave are blocked on sending/receiving message. And > > > this in-turn allows for shutting down of virtqueues, hence resolving > > > the deadlock. > > > > Is the new message necessary? > > Hi Stefan, > > It probably is not necessary but it feels like a cleaner and simpler > solution. > > There slave is a separate channel from virtqueues. And if device is > stopping, we want to make sure there are no pending requests in > slave channel. It is possible that virtqueue shutodwn is successful > but other entity could still be sending messages on slave channel. In > that case, shall we say device shutdown complete and stop polling > slave channel? > > IOW, the way we have explicit protocol messages to shutdown each > vring, it makes sense to have explicit protocol message to shutdown > slave channel as well. Right now there is no mechanism to do that. > Thinking more from "slave" point of view. It might have a separate thread to send messages on slave channel. It has no idea when to stop sending messages on slave channel, as there is no mechanism to shutdown slave. Only method seems to be that master will close slave fd and that will close connection. That means if a device is being stopped (and not removed), then it is possible virtqueues got stopped but a thread in slave is blocked on slave channel (recvmsg()) and expecting a response back. That's why I think enhancing protocol to explicitly shutdown slave channel makes sense. Instead of assuming that stopping a virtuqueue should automatically mean slave channel is stopped. Vivek > IMHO, introducing an explicit mechanism in protocol to shutdown > slave channel feels better as compared to say keep on serving > slave channel in parallel while waiting for virtuqueues to shutdown > and stop serving slave as soon as last virtuqueue is stopped (there > could still be pending slave message right after last virtuqueue > got shutdown). > > > > How about letting QEMU handle slave fd > > activity while waiting for virtqueues to stop instead? > > > > In other words, QEMU should monitor both the UNIX domain socket and the > > slave fd after it has sent VHOST_USER_GET_VRING_BASE and is awaiting the > > response. > > I can give it a try. If there is a strong preference for this solution. > I guess I can patch vhost_user_get_vring_base() to poll and serve both > unix domain socket and slave fd. > > But at this point of time I do think that adding a mechanism to shutodwn > slave channel (the way we have mechanism to shutdown vring), sounds > better from design point of view. > > Vivek
On Fri, Jan 29, 2021 at 10:11:55AM -0500, Vivek Goyal wrote: > On Fri, Jan 29, 2021 at 09:16:00AM -0500, Vivek Goyal wrote: > > On Thu, Jan 28, 2021 at 04:52:34PM +0000, Stefan Hajnoczi wrote: > > > On Mon, Jan 25, 2021 at 01:01:13PM -0500, Vivek Goyal wrote: > > > > Currently we don't have a mechanism to flush slave channel while shutting > > > > down vhost-user device and that can result a deadlock. Consider following > > > > scenario. > > > > > > > > 1. Slave gets a request from guest on virtqueue (say index 1, vq1), to map > > > > a portion of file in qemu address space. > > > > > > > > 2. Thread serving vq1 receives this request and sends a message to qemu on > > > > slave channel/fd and gets blocked in waiting for a response from qemu. > > > > > > > > 3. In the mean time, user does "echo b > /proc/sysrq-trigger" in guest. This > > > > leads to qemu reset and ultimately in main thread we end up in > > > > vhost_dev_stop() which is trying to shutdown virtqueues for the device. > > > > > > > > 4. Slave gets VHOST_USER_GET_VRING_BASE message to shutdown a virtqueue on > > > > unix socket being used for communication. > > > > > > > > 5. Slave tries to shutdown the thread serving vq1 and waits for it to > > > > terminate. But vq1 thread can't terminate because it is waiting for > > > > a response from qemu on slave_fd. And qemu is not processing slave_fd > > > > anymore as qemu is ressing (and not running main event loop anymore) > > > > and is waiting for a response to VHOST_USER_GET_VRING_BASE. > > > > > > > > So we have a deadlock. Qemu is waiting on slave to response to > > > > VHOST_USER_GET_VRING_BASE message and slave is waiting on qemu to > > > > respond to request it sent on slave_fd. > > > > > > > > I can reliably reproduce this race with virtio-fs. > > > > > > > > Hence here is the proposal to solve this problem. Enhance vhost-user > > > > protocol to properly shutdown slave_fd channel. And if there are pending > > > > requests, flush the channel completely before sending the request to > > > > shutdown virtqueues. > > > > > > > > New workflow to shutdown slave channel is. > > > > > > > > - Qemu sends VHOST_USER_STOP_SLAVE_CHANNEL request to slave. It waits > > > > for an reply from guest. > > > > > > > > - Then qemu sits in a tight loop waiting for > > > > VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message from slave on slave_fd. > > > > And while waiting for this message, qemu continues to process requests > > > > on slave_fd to flush any pending requests. This will unblock threads > > > > in slave and allow slave to shutdown slave channel. > > > > > > > > - Once qemu gets VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message, it knows > > > > no more requests will come on slave_fd and it can continue to shutdown > > > > virtqueues now. > > > > > > > > - Once device starts again, qemu will send VHOST_USER_START_SLAVE_CHANNEL > > > > message to slave to open the slave channel and receive requests. > > > > > > > > IOW, this allows for proper shutdown of slave channel, making sure > > > > no threads in slave are blocked on sending/receiving message. And > > > > this in-turn allows for shutting down of virtqueues, hence resolving > > > > the deadlock. > > > > > > Is the new message necessary? > > > > Hi Stefan, > > > > It probably is not necessary but it feels like a cleaner and simpler > > solution. > > > > There slave is a separate channel from virtqueues. And if device is > > stopping, we want to make sure there are no pending requests in > > slave channel. It is possible that virtqueue shutodwn is successful > > but other entity could still be sending messages on slave channel. In > > that case, shall we say device shutdown complete and stop polling > > slave channel? > > > > IOW, the way we have explicit protocol messages to shutdown each > > vring, it makes sense to have explicit protocol message to shutdown > > slave channel as well. Right now there is no mechanism to do that. > > > > Thinking more from "slave" point of view. It might have a separate thread > to send messages on slave channel. It has no idea when to stop sending > messages on slave channel, as there is no mechanism to shutdown slave. > Only method seems to be that master will close slave fd and that > will close connection. > > That means if a device is being stopped (and not removed), then it > is possible virtqueues got stopped but a thread in slave is blocked > on slave channel (recvmsg()) and expecting a response back. > > That's why I think enhancing protocol to explicitly shutdown slave > channel makes sense. Instead of assuming that stopping a virtuqueue > should automatically mean slave channel is stopped. This deadlock can occur during any vhost-user protocol message, not just shutdown: When QEMU's vhost-user master implementation sends a vhost-user protocol message, vhost_user_read() does a "blocking" read during which slave_fd is not monitored by QEMU. So the problem is more general and is not fully solved by an explicit shutdown message. If the slave needs slave channel reply in order to process a vhost-user protocol message then a deadlock can occur. This can be solved by monitoring both the vhost-user fd and slave_fd in vhost_user_read(). I don't think there is a need to shutdown the slave channel BTW. Stefan
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c index 867cac034f..56be61d4e2 100644 --- a/hw/virtio/vhost-user.c +++ b/hw/virtio/vhost-user.c @@ -80,6 +80,7 @@ enum VhostUserProtocolFeature { VHOST_USER_PROTOCOL_F_RESET_DEVICE = 13, /* Feature 14 reserved for VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS. */ VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS = 15, + VHOST_USER_PROTOCOL_F_SLAVE_CH_START_STOP = 16, VHOST_USER_PROTOCOL_F_MAX }; @@ -125,6 +126,8 @@ typedef enum VhostUserRequest { VHOST_USER_GET_MAX_MEM_SLOTS = 36, VHOST_USER_ADD_MEM_REG = 37, VHOST_USER_REM_MEM_REG = 38, + VHOST_USER_START_SLAVE_CHANNEL = 39, + VHOST_USER_STOP_SLAVE_CHANNEL = 40, VHOST_USER_MAX } VhostUserRequest; @@ -139,6 +142,7 @@ typedef enum VhostUserSlaveRequest { VHOST_USER_SLAVE_FS_UNMAP = 7, VHOST_USER_SLAVE_FS_SYNC = 8, VHOST_USER_SLAVE_FS_IO = 9, + VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE = 10, VHOST_USER_SLAVE_MAX } VhostUserSlaveRequest; @@ -246,6 +250,7 @@ struct vhost_user { /* Shared between vhost devs of the same virtio device */ VhostUserState *user; int slave_fd; + bool slave_channel_open; NotifierWithReturn postcopy_notifier; struct PostCopyFD postcopy_fd; uint64_t postcopy_client_bases[VHOST_USER_MAX_RAM_SLOTS]; @@ -1511,6 +1516,10 @@ static int do_slave_read(void *opaque) ret = vhost_user_fs_slave_io(dev, &payload.fs, fd[0]); break; #endif + case VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE: + u->slave_channel_open = false; + ret = 0; + break; default: error_report("Received unexpected msg type: %d.", hdr.request); ret = (uint64_t)-EINVAL; @@ -1580,6 +1589,70 @@ static void slave_read(void *opaque) do_slave_read(opaque); } +static int vhost_start_slave_channel(struct vhost_dev *dev) +{ + struct vhost_user *u = dev->opaque; + VhostUserMsg msg = { + .hdr.request = VHOST_USER_START_SLAVE_CHANNEL, + .hdr.flags = VHOST_USER_VERSION | VHOST_USER_NEED_REPLY_MASK, + }; + int ret = 0; + + if (!virtio_has_feature(dev->protocol_features, + VHOST_USER_PROTOCOL_F_SLAVE_CH_START_STOP)) { + return 0; + } + + ret = vhost_user_write(dev, &msg, NULL, 0); + if (ret) { + return ret; + } + + ret = process_message_reply(dev, &msg); + if (ret) + return ret; + + u->slave_channel_open = true; + return ret; +} + +static int vhost_stop_slave_channel(struct vhost_dev *dev) +{ + struct vhost_user *u = dev->opaque; + VhostUserMsg msg = { + .hdr.request = VHOST_USER_STOP_SLAVE_CHANNEL, + .hdr.flags = VHOST_USER_VERSION | VHOST_USER_NEED_REPLY_MASK, + }; + int ret = 0; + + if (!virtio_has_feature(dev->protocol_features, + VHOST_USER_PROTOCOL_F_SLAVE_CH_START_STOP)) { + return 0; + } + + ret = vhost_user_write(dev, &msg, NULL, 0); + if (ret) { + return ret; + } + + ret = process_message_reply(dev, &msg); + if (ret) { + return ret; + } + + /* + * Wait for flush finish message from slave. And continue to process + * slave messages till we get flush finish. + */ + while (u->slave_channel_open) { + ret = do_slave_read(dev); + if (ret) + break; + } + + return ret; +} + static int vhost_setup_slave_channel(struct vhost_dev *dev) { VhostUserMsg msg = { @@ -1860,6 +1933,7 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque) u = g_new0(struct vhost_user, 1); u->user = opaque; u->slave_fd = -1; + u->slave_channel_open = false; u->dev = dev; dev->opaque = u; @@ -1934,6 +2008,17 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque) u->user->memory_slots = MIN(ram_slots, VHOST_USER_MAX_RAM_SLOTS); } + + if (virtio_has_feature(dev->protocol_features, + VHOST_USER_PROTOCOL_F_SLAVE_CH_START_STOP) && + !(virtio_has_feature(dev->protocol_features, + VHOST_USER_PROTOCOL_F_SLAVE_REQ) && + virtio_has_feature(dev->protocol_features, + VHOST_USER_PROTOCOL_F_REPLY_ACK))) { + error_report("Slave channel start/stop support requires reply-ack" + " and slave-req protocol features."); + return -1; + } } if (dev->migration_blocker == NULL && @@ -1949,6 +2034,10 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque) if (err < 0) { return err; } + err = vhost_start_slave_channel(dev); + if (err < 0) { + return err; + } } u->postcopy_notifier.notify = vhost_user_postcopy_notifier; @@ -2408,6 +2497,24 @@ void vhost_user_cleanup(VhostUserState *user) user->chr = NULL; } +static int vhost_user_dev_start(struct vhost_dev *dev, bool started) +{ + int ret; + + if (!started) { + ret = vhost_stop_slave_channel(dev); + if (ret < 0) { + return ret; + } + } else { + ret = vhost_start_slave_channel(dev); + if (ret < 0) { + return ret; + } + } + return 0; +} + const VhostOps user_ops = { .backend_type = VHOST_BACKEND_TYPE_USER, .vhost_backend_init = vhost_user_backend_init, @@ -2434,6 +2541,7 @@ const VhostOps user_ops = { .vhost_net_set_mtu = vhost_user_net_set_mtu, .vhost_set_iotlb_callback = vhost_user_set_iotlb_callback, .vhost_send_device_iotlb_msg = vhost_user_send_device_iotlb_msg, + .vhost_dev_start = vhost_user_dev_start, .vhost_get_config = vhost_user_get_config, .vhost_set_config = vhost_user_set_config, .vhost_crypto_create_session = vhost_user_crypto_create_session,
Currently we don't have a mechanism to flush slave channel while shutting down vhost-user device and that can result a deadlock. Consider following scenario. 1. Slave gets a request from guest on virtqueue (say index 1, vq1), to map a portion of file in qemu address space. 2. Thread serving vq1 receives this request and sends a message to qemu on slave channel/fd and gets blocked in waiting for a response from qemu. 3. In the mean time, user does "echo b > /proc/sysrq-trigger" in guest. This leads to qemu reset and ultimately in main thread we end up in vhost_dev_stop() which is trying to shutdown virtqueues for the device. 4. Slave gets VHOST_USER_GET_VRING_BASE message to shutdown a virtqueue on unix socket being used for communication. 5. Slave tries to shutdown the thread serving vq1 and waits for it to terminate. But vq1 thread can't terminate because it is waiting for a response from qemu on slave_fd. And qemu is not processing slave_fd anymore as qemu is ressing (and not running main event loop anymore) and is waiting for a response to VHOST_USER_GET_VRING_BASE. So we have a deadlock. Qemu is waiting on slave to response to VHOST_USER_GET_VRING_BASE message and slave is waiting on qemu to respond to request it sent on slave_fd. I can reliably reproduce this race with virtio-fs. Hence here is the proposal to solve this problem. Enhance vhost-user protocol to properly shutdown slave_fd channel. And if there are pending requests, flush the channel completely before sending the request to shutdown virtqueues. New workflow to shutdown slave channel is. - Qemu sends VHOST_USER_STOP_SLAVE_CHANNEL request to slave. It waits for an reply from guest. - Then qemu sits in a tight loop waiting for VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message from slave on slave_fd. And while waiting for this message, qemu continues to process requests on slave_fd to flush any pending requests. This will unblock threads in slave and allow slave to shutdown slave channel. - Once qemu gets VHOST_USER_SLAVE_STOP_CHANNEL_COMPLETE message, it knows no more requests will come on slave_fd and it can continue to shutdown virtqueues now. - Once device starts again, qemu will send VHOST_USER_START_SLAVE_CHANNEL message to slave to open the slave channel and receive requests. IOW, this allows for proper shutdown of slave channel, making sure no threads in slave are blocked on sending/receiving message. And this in-turn allows for shutting down of virtqueues, hence resolving the deadlock. Signed-off-by: Vivek Goyal <vgoyal@redhat.com> --- hw/virtio/vhost-user.c | 108 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 108 insertions(+)