[PATCH 1/5] block/io: introduce bdrv_try_mark_request_serialising

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Introduce a function to mark the request serialising only if there are
no conflicting request to wait for.

The function is static, so mark it unused. The attribute is to be
dropped in the next commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/io.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 51 insertions(+), 7 deletions(-)

diff --git a/block/io.c b/block/io.c
index df8f2a98d4..60cee1d759 100644
--- a/block/io.c
+++ b/block/io.c
@@ -727,10 +727,11 @@ static bool tracked_request_overlaps(BdrvTrackedRequest *req,
     return true;
 }
 
-static bool coroutine_fn
-bdrv_wait_serialising_requests_locked(BlockDriverState *bs,
-                                      BdrvTrackedRequest *self)
+/* Called with self->bs->reqs_lock locked */
+static bool coroutine_fn wait_or_find_conflicts(BdrvTrackedRequest *self,
+                                                bool wait)
 {
+    BlockDriverState *bs = self->bs;
     BdrvTrackedRequest *req;
     bool retry;
     bool waited = false;
@@ -754,6 +755,9 @@ bdrv_wait_serialising_requests_locked(BlockDriverState *bs,
                  * will wait for us as soon as it wakes up, then just go on
                  * (instead of producing a deadlock in the former case). */
                 if (!req->waiting_for) {
+                    if (!wait) {
+                        return true;
+                    }
                     self->waiting_for = req;
                     qemu_co_queue_wait(&req->wait_queue, &bs->reqs_lock);
                     self->waiting_for = NULL;
@@ -767,13 +771,18 @@ bdrv_wait_serialising_requests_locked(BlockDriverState *bs,
     return waited;
 }
 
-bool bdrv_mark_request_serialising(BdrvTrackedRequest *req, uint64_t align)
+/* Return true on success, false on fail. Always success with blocking = true */
+static bool bdrv_do_mark_request_serialising(BdrvTrackedRequest *req,
+        uint64_t align, bool blocking, bool *waited)
 {
     BlockDriverState *bs = req->bs;
     int64_t overlap_offset = req->offset & ~(align - 1);
     uint64_t overlap_bytes = ROUND_UP(req->offset + req->bytes, align)
                                - overlap_offset;
-    bool waited;
+    int64_t old_overlap_offset = req->overlap_offset;
+    uint64_t old_overlap_bytes = req->overlap_bytes;
+    bool old_serializing = req->serialising;
+    bool found;
 
     qemu_co_mutex_lock(&bs->reqs_lock);
     if (!req->serialising) {
@@ -783,11 +792,46 @@ bool bdrv_mark_request_serialising(BdrvTrackedRequest *req, uint64_t align)
 
     req->overlap_offset = MIN(req->overlap_offset, overlap_offset);
     req->overlap_bytes = MAX(req->overlap_bytes, overlap_bytes);
-    waited = bdrv_wait_serialising_requests_locked(bs, req);
+    found = wait_or_find_conflicts(req, blocking);
+
+    if (!blocking && found) {
+        req->overlap_offset = old_overlap_offset;
+        req->overlap_bytes = old_overlap_bytes;
+        if (!old_serializing) {
+            atomic_dec(&req->bs->serialising_in_flight);
+            req->serialising = false;
+        }
+    }
+
+    *waited = found && blocking;
+
     qemu_co_mutex_unlock(&bs->reqs_lock);
+
+    return blocking || !found;
+}
+
+/* Return true if had to wait for conflicts */
+bool bdrv_mark_request_serialising(BdrvTrackedRequest *req, uint64_t align)
+{
+    bool waited;
+    bool success = bdrv_do_mark_request_serialising(req, align, true, &waited);
+
+    assert(success);
     return waited;
 }
 
+/* Return true on success, false if there are some conflicts */
+__attribute__ ((unused))
+static bool bdrv_try_mark_request_serialising(BdrvTrackedRequest *req,
+                                              uint64_t align)
+{
+    bool waited;
+    bool success = bdrv_do_mark_request_serialising(req, align, false, &waited);
+
+    assert(!waited);
+    return success;
+}
+
 /**
  * Return the tracked request on @bs for the current coroutine, or
  * NULL if there is none.
@@ -865,7 +909,7 @@ static bool coroutine_fn bdrv_wait_serialising_requests(BdrvTrackedRequest *self
     }
 
     qemu_co_mutex_lock(&bs->reqs_lock);
-    waited = bdrv_wait_serialising_requests_locked(bs, self);
+    waited = wait_or_find_conflicts(self, true);
     qemu_co_mutex_unlock(&bs->reqs_lock);
 
     return waited;
-- 
2.18.0