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block/export: wait for vhost-user-blk requests when draining

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Each vhost-user-blk request runs in a coroutine. When the BlockBackend
enters a drained section we need to enter a quiescent state. Currently
any in-flight requests race with bdrv_drained_begin() because it is
unaware of vhost-user-blk requests.

When blk_co_preadv/pwritev()/etc returns it wakes the
bdrv_drained_begin() thread but vhost-user-blk request processing has
not yet finished. The request coroutine continues executing while the
main loop thread thinks it is in a drained section.

One example where this is unsafe is for blk_set_aio_context() where
bdrv_drained_begin() is called before .aio_context_detached() and
.aio_context_attach(). If request coroutines are still running after
bdrv_drained_begin(), then the AioContext could change underneath them
and they race with new requests processed in the new AioContext. This
could lead to virtqueue corruption, for example.

(This example is theoretical, I came across this while reading the
code and have not tried to reproduce it.)

It's easy to make bdrv_drained_begin() wait for in-flight requests: add
a .drained_poll() callback that checks the VuServer's in-flight counter.
VuServer just needs an API that returns true when there are requests in
flight. The in-flight counter needs to be atomic.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-7-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2023-05-16 15:02:24 -04:00 • committed by Kevin Wolf
parent 75d33e8525
commit 8f5e9a8ee1
3 changed files with 28 additions and 7 deletions
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4
include/qemu/vhost-user-server.h
View file

@ -40,8 +40,9 @@ typedef struct {
int max_queues;
const VuDevIface *vu_iface;
unsigned int in_flight; /* atomic */
/* Protected by ctx lock */
unsigned int in_flight;
bool wait_idle;
VuDev vu_dev;
QIOChannel *ioc; /* The I/O channel with the client */
@ -62,6 +63,7 @@ void vhost_user_server_stop(VuServer *server);
void vhost_user_server_inc_in_flight(VuServer *server);
void vhost_user_server_dec_in_flight(VuServer *server);
bool vhost_user_server_has_in_flight(VuServer *server);
void vhost_user_server_attach_aio_context(VuServer *server, AioContext *ctx);
void vhost_user_server_detach_aio_context(VuServer *server);