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[Qemu-devel] [PATCH v3 19/29] block: Allow wait_serialising_requests() a


From: Kevin Wolf
Subject: [Qemu-devel] [PATCH v3 19/29] block: Allow wait_serialising_requests() at any point
Date: Fri, 17 Jan 2014 15:15:09 +0100

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 <address@hidden>
Reviewed-by: Max Reitz <address@hidden>
---
 block.c                   | 13 ++++++++++---
 include/block/block_int.h |  2 ++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index e72966a..55e8c69 100644
--- a/block.c
+++ b/block.c
@@ -2148,9 +2148,16 @@ 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 (indirectly) waiting for us, or
+                 * 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) {
+                    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 ccd2c68..fdf0e0b 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




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