throttle-groups: protect throttled requests with a CoMutex

Another possibility is to use tg->lock, which we're holding anyway in
both schedule_next_request and throttle_group_co_io_limits_intercept.
This would require open-coding the CoQueue however, so I've chosen this
alternative.
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170605123908.18777-10-pbonzini@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>

block/block-backend.c

@@ -216,6 +216,7 @@ BlockBackend *blk_new(uint64_t perm, uint64_t shared_perm)
     blk->shared_perm = shared_perm;
     blk_set_enable_write_cache(blk, true);
 
+    qemu_co_mutex_init(&blk->public.throttled_reqs_lock);
     qemu_co_queue_init(&blk->public.throttled_reqs[0]);
     qemu_co_queue_init(&blk->public.throttled_reqs[1]);
 

block/throttle-groups.c

@@ -270,8 +270,13 @@ static bool coroutine_fn throttle_group_co_restart_queue(BlockBackend *blk,
                                                          bool is_write)
 {
     BlockBackendPublic *blkp = blk_get_public(blk);
+    bool ret;
 
-    return qemu_co_queue_next(&blkp->throttled_reqs[is_write]);
+    qemu_co_mutex_lock(&blkp->throttled_reqs_lock);
+    ret = qemu_co_queue_next(&blkp->throttled_reqs[is_write]);
+    qemu_co_mutex_unlock(&blkp->throttled_reqs_lock);
+
+    return ret;
 }
 
 /* Look for the next pending I/O request and schedule it.
@@ -340,7 +345,10 @@ void coroutine_fn throttle_group_co_io_limits_intercept(BlockBackend *blk,
     if (must_wait || blkp->pending_reqs[is_write]) {
         blkp->pending_reqs[is_write]++;
         qemu_mutex_unlock(&tg->lock);
-        qemu_co_queue_wait(&blkp->throttled_reqs[is_write], NULL);
+        qemu_co_mutex_lock(&blkp->throttled_reqs_lock);
+        qemu_co_queue_wait(&blkp->throttled_reqs[is_write],
+                           &blkp->throttled_reqs_lock);
+        qemu_co_mutex_unlock(&blkp->throttled_reqs_lock);
         qemu_mutex_lock(&tg->lock);
         blkp->pending_reqs[is_write]--;
     }

include/sysemu/block-backend.h

@@ -72,11 +72,8 @@ typedef struct BlockDevOps {
  * fields that must be public. This is in particular for QLIST_ENTRY() and
  * friends so that BlockBackends can be kept in lists outside block-backend.c */
 typedef struct BlockBackendPublic {
-    /* I/O throttling has its own locking, but also some fields are
-     * protected by the AioContext lock.
-     */
-
-    /* Protected by AioContext lock.  */
+    /* throttled_reqs_lock protects the CoQueues for throttled requests.  */
+    CoMutex      throttled_reqs_lock;
     CoQueue      throttled_reqs[2];
 
     /* Nonzero if the I/O limits are currently being ignored; generally