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 <kw...@redhat.com>
---
block.c | 26 +++++++++++++++++++++++---
include/block/block_int.h | 2 ++
2 files changed, 25 insertions(+), 3 deletions(-)
diff --git a/block.c b/block.c
index 73bba47..8a0225d 100644
--- a/block.c
+++ b/block.c
@@ -2123,6 +2123,20 @@ static bool tracked_request_overlaps(BdrvTrackedRequest
*req,
return true;
}
+static bool coroutine_fn is_waiting_for(BdrvTrackedRequest *waiting,
+ BdrvTrackedRequest *waited_for)
+{
+ BdrvTrackedRequest *req;
+
+ for (req = waiting; req != NULL; req = req->waiting_for) {
+ if (req == waited_for) {
+ return true;
+ }
+ }
+
+ return false;
+}
+
static void coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self)
{
BlockDriverState *bs = self->bs;
@@ -2148,9 +2162,15 @@ static void coroutine_fn
wait_serialising_requests(BdrvTrackedRequest *self)
*/
assert(qemu_coroutine_self() != req->co);
- qemu_co_queue_wait(&req->wait_queue);
- retry = true;
- break;
+ /* If the request is already (indircetly) waiting for us, then
+ * just go on instead of producing a deadlock. */
+ if (!is_waiting_for(req, self)) {
+ self->waiting_for = req;
+ qemu_co_queue_wait(&req->wait_queue);
+ self->waiting_for = NULL;
+ retry = true;
+ break;
+ }
}
}
} while (retry);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 1aee02b..0f7fcef 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 {
--
1.8.1.4
