block: Allow wait_serialising_requests() at any point

We can only have a single wait_serialising_requests() call per request
because otherwise we can run into deadlocks where requests are waiting
for each other. The same is true when wait_serialising_requests() is not
at the very beginning of a request, so that other requests can be issued
between the start of the tracking and wait_serialising_requests().

Fix this by changing wait_serialising_requests() to ignore requests that
are already (directly or indirectly) waiting for the calling request.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 0ee955cb76..0bcf1c9b8c 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -68,6 +68,8 @@ typedef struct BdrvTrackedRequest {
     QLIST_ENTRY(BdrvTrackedRequest) list;
     Coroutine *co; /* owner, used for deadlock detection */
     CoQueue wait_queue; /* coroutines blocked on this request */
+
+    struct BdrvTrackedRequest *waiting_for;
 } BdrvTrackedRequest;
 
 struct BlockDriver {