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nbd: make it thread-safe, fix qcow2 over nbd

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NBD is not thread safe, because it accesses s->in_flight without
a CoMutex.  Fixing this will be required for multiqueue.
CoQueue doesn't have spurious wakeups but, when another coroutine can
run between qemu_co_queue_next's wakeup and qemu_co_queue_wait's
re-locking of the mutex, the wait condition can become false and
a loop is necessary.

In fact, it turns out that the loop is necessary even without this
multi-threaded scenario.  A particular sequence of coroutine wakeups
is happening ~80% of the time when starting a guest with qcow2 image
served over NBD (i.e. qemu-nbd --format=raw, and QEMU's -drive option
has -format=qcow2).  This patch fixes that issue too.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2017-06-01 12:44:56 +02:00
parent b8158192fa
commit 6bdcc018a6
1 changed files with 9 additions and 21 deletions
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30
block/nbd-client.c
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@ -119,6 +119,10 @@ static int nbd_co_send_request(BlockDriverState *bs,
int rc, ret, i;
qemu_co_mutex_lock(&s->send_mutex);
while (s->in_flight == MAX_NBD_REQUESTS) {
qemu_co_queue_wait(&s->free_sema, &s->send_mutex);
}
s->in_flight++;
for (i = 0; i < MAX_NBD_REQUESTS; i++) {
if (s->recv_coroutine[i] == NULL) {
@ -181,20 +185,6 @@ static void nbd_co_receive_reply(NBDClientSession *s,
}
}
static void nbd_coroutine_start(NBDClientSession *s,
NBDRequest *request)
{
/* Poor man semaphore. The free_sema is locked when no other request
* can be accepted, and unlocked after receiving one reply. */
if (s->in_flight == MAX_NBD_REQUESTS) {
qemu_co_queue_wait(&s->free_sema, NULL);
assert(s->in_flight < MAX_NBD_REQUESTS);
}
s->in_flight++;
/* s->recv_coroutine[i] is set as soon as we get the send_lock. */
}
static void nbd_coroutine_end(BlockDriverState *bs,
NBDRequest *request)
{
@ -202,13 +192,16 @@ static void nbd_coroutine_end(BlockDriverState *bs,
int i = HANDLE_TO_INDEX(s, request->handle);
s->recv_coroutine[i] = NULL;
s->in_flight--;
qemu_co_queue_next(&s->free_sema);
/* Kick the read_reply_co to get the next reply. */
if (s->read_reply_co) {
aio_co_wake(s->read_reply_co);
}
qemu_co_mutex_lock(&s->send_mutex);
s->in_flight--;
qemu_co_queue_next(&s->free_sema);
qemu_co_mutex_unlock(&s->send_mutex);
}
int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,
@ -226,7 +219,6 @@ int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,
assert(bytes <= NBD_MAX_BUFFER_SIZE);
assert(!flags);
nbd_coroutine_start(client, &request);
ret = nbd_co_send_request(bs, &request, NULL);
if (ret < 0) {
reply.error = -ret;
@ -256,7 +248,6 @@ int nbd_client_co_pwritev(BlockDriverState *bs, uint64_t offset,
assert(bytes <= NBD_MAX_BUFFER_SIZE);
nbd_coroutine_start(client, &request);
ret = nbd_co_send_request(bs, &request, qiov);
if (ret < 0) {
reply.error = -ret;
@ -291,7 +282,6 @@ int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
request.flags |= NBD_CMD_FLAG_NO_HOLE;
}
nbd_coroutine_start(client, &request);
ret = nbd_co_send_request(bs, &request, NULL);
if (ret < 0) {
reply.error = -ret;
@ -316,7 +306,6 @@ int nbd_client_co_flush(BlockDriverState *bs)
request.from = 0;
request.len = 0;
nbd_coroutine_start(client, &request);
ret = nbd_co_send_request(bs, &request, NULL);
if (ret < 0) {
reply.error = -ret;
@ -342,7 +331,6 @@ int nbd_client_co_pdiscard(BlockDriverState *bs, int64_t offset, int count)
return 0;
}
nbd_coroutine_start(client, &request);
ret = nbd_co_send_request(bs, &request, NULL);
if (ret < 0) {
reply.error = -ret;