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nbd/server: Fix race in draining the export

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When draining an NBD export, nbd_drained_begin() first sets
client->quiescing so that nbd_client_receive_next_request() won't start
any new request coroutines. Then nbd_drained_poll() tries to makes sure
that we wait for any existing request coroutines by checking that
client->nb_requests has become 0.

However, there is a small window between creating a new request
coroutine and increasing client->nb_requests. If a coroutine is in this
state, it won't be waited for and drain returns too early.

In the context of switching to a different AioContext, this means that
blk_aio_attached() will see client->recv_coroutine != NULL and fail its
assertion.

Fix this by increasing client->nb_requests immediately when starting the
coroutine. Doing this after the checks if we should create a new
coroutine is okay because client->lock is held.

Cc: qemu-stable@nongnu.org
Fixes: fd6afc501a ("nbd/server: Use drained block ops to quiesce the server")
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20240314165825.40261-2-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Kevin Wolf 2024-03-14 17:58:24 +01:00
parent ae5a40e858
commit 9c707525cb
1 changed files with 7 additions and 8 deletions
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13
nbd/server.c
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@ -3007,8 +3007,8 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
/* Owns a reference to the NBDClient passed as opaque. */
static coroutine_fn void nbd_trip(void *opaque)
{
NBDClient *client = opaque;
NBDRequestData *req = NULL;
NBDRequestData *req = opaque;
NBDClient *client = req->client;
NBDRequest request = { 0 }; /* GCC thinks it can be used uninitialized */
int ret;
Error *local_err = NULL;
@ -3037,8 +3037,6 @@ static coroutine_fn void nbd_trip(void *opaque)
goto done;
}
req = nbd_request_get(client);
/*
* nbd_co_receive_request() returns -EAGAIN when nbd_drained_begin() has
* set client->quiescing but by the time we get back nbd_drained_end() may
@ -3112,9 +3110,7 @@ static coroutine_fn void nbd_trip(void *opaque)
}
done:
if (req) {
nbd_request_put(req);
}
qemu_mutex_unlock(&client->lock);
@ -3143,10 +3139,13 @@ disconnect:
*/
static void nbd_client_receive_next_request(NBDClient *client)
{
NBDRequestData *req;
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);
req = nbd_request_get(client);
client->recv_coroutine = qemu_coroutine_create(nbd_trip, req);
aio_co_schedule(client->exp->common.ctx, client->recv_coroutine);
}
}