122 lines
4.1 KiB
Diff
122 lines
4.1 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Thu, 1 Jun 2017 12:44:56 +0200
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Subject: [PATCH] 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
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a CoMutex. Fixing this will be required for multiqueue.
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CoQueue doesn't have spurious wakeups but, when another coroutine can
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run between qemu_co_queue_next's wakeup and qemu_co_queue_wait's
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re-locking of the mutex, the wait condition can become false and
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a loop is necessary.
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In fact, it turns out that the loop is necessary even without this
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multi-threaded scenario. A particular sequence of coroutine wakeups
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is happening ~80% of the time when starting a guest with qcow2 image
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served over NBD (i.e. qemu-nbd --format=raw, and QEMU's -drive option
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has -format=qcow2). This patch fixes that issue too.
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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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---
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block/nbd-client.c | 30 +++++++++---------------------
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1 file changed, 9 insertions(+), 21 deletions(-)
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diff --git a/block/nbd-client.c b/block/nbd-client.c
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index 56eb0e2e16..282679b4f8 100644
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--- a/block/nbd-client.c
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+++ b/block/nbd-client.c
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@@ -114,6 +114,10 @@ static int nbd_co_send_request(BlockDriverState *bs,
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int rc, ret, i;
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qemu_co_mutex_lock(&s->send_mutex);
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+ while (s->in_flight == MAX_NBD_REQUESTS) {
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+ qemu_co_queue_wait(&s->free_sema, &s->send_mutex);
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+ }
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+ s->in_flight++;
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for (i = 0; i < MAX_NBD_REQUESTS; i++) {
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if (s->recv_coroutine[i] == NULL) {
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@@ -176,20 +180,6 @@ static void nbd_co_receive_reply(NBDClientSession *s,
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}
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}
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-static void nbd_coroutine_start(NBDClientSession *s,
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- NBDRequest *request)
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-{
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- /* Poor man semaphore. The free_sema is locked when no other request
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- * can be accepted, and unlocked after receiving one reply. */
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- if (s->in_flight == MAX_NBD_REQUESTS) {
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- qemu_co_queue_wait(&s->free_sema, NULL);
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- assert(s->in_flight < MAX_NBD_REQUESTS);
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- }
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- s->in_flight++;
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-
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- /* s->recv_coroutine[i] is set as soon as we get the send_lock. */
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-}
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-
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static void nbd_coroutine_end(BlockDriverState *bs,
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NBDRequest *request)
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{
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@@ -197,13 +187,16 @@ static void nbd_coroutine_end(BlockDriverState *bs,
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int i = HANDLE_TO_INDEX(s, request->handle);
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s->recv_coroutine[i] = NULL;
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- s->in_flight--;
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- qemu_co_queue_next(&s->free_sema);
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/* Kick the read_reply_co to get the next reply. */
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if (s->read_reply_co) {
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aio_co_wake(s->read_reply_co);
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}
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+
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+ qemu_co_mutex_lock(&s->send_mutex);
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+ s->in_flight--;
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+ qemu_co_queue_next(&s->free_sema);
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+ qemu_co_mutex_unlock(&s->send_mutex);
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}
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int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,
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@@ -221,7 +214,6 @@ int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,
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assert(bytes <= NBD_MAX_BUFFER_SIZE);
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assert(!flags);
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- nbd_coroutine_start(client, &request);
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ret = nbd_co_send_request(bs, &request, NULL);
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if (ret < 0) {
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reply.error = -ret;
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@@ -251,7 +243,6 @@ int nbd_client_co_pwritev(BlockDriverState *bs, uint64_t offset,
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assert(bytes <= NBD_MAX_BUFFER_SIZE);
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- nbd_coroutine_start(client, &request);
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ret = nbd_co_send_request(bs, &request, qiov);
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if (ret < 0) {
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reply.error = -ret;
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@@ -286,7 +277,6 @@ int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
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request.flags |= NBD_CMD_FLAG_NO_HOLE;
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}
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- nbd_coroutine_start(client, &request);
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ret = nbd_co_send_request(bs, &request, NULL);
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if (ret < 0) {
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reply.error = -ret;
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@@ -311,7 +301,6 @@ int nbd_client_co_flush(BlockDriverState *bs)
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request.from = 0;
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request.len = 0;
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- nbd_coroutine_start(client, &request);
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ret = nbd_co_send_request(bs, &request, NULL);
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if (ret < 0) {
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reply.error = -ret;
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@@ -337,7 +326,6 @@ int nbd_client_co_pdiscard(BlockDriverState *bs, int64_t offset, int count)
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return 0;
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}
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- nbd_coroutine_start(client, &request);
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ret = nbd_co_send_request(bs, &request, NULL);
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if (ret < 0) {
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reply.error = -ret;
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--
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2.11.0
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