Patchwork [2/3] block: wait_for_overlapping_requests() deadlock detection

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Submitter Stefan Hajnoczi
Date Nov. 30, 2011, 12:23 p.m.
Message ID <1322655824-31778-3-git-send-email-stefanha@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/128493/
State New
Headers show

Comments

Stefan Hajnoczi - Nov. 30, 2011, 12:23 p.m.
Debugging a reentrant request deadlock was fun but in the future we need
a quick and obvious way of detecting such bugs.  Add an assert that
checks we are not about to deadlock when waiting for another request.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 block.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

Patch

diff --git a/block.c b/block.c
index 4310eb5..37a96f3 100644
--- a/block.c
+++ b/block.c
@@ -1099,6 +1099,7 @@  struct BdrvTrackedRequest {
     int nb_sectors;
     bool is_write;
     QLIST_ENTRY(BdrvTrackedRequest) list;
+    Coroutine *co; /* owner, used for deadlock detection */
     CoQueue wait_queue; /* coroutines blocked on this request */
 };
 
@@ -1126,6 +1127,7 @@  static void tracked_request_begin(BdrvTrackedRequest *req,
         .sector_num = sector_num,
         .nb_sectors = nb_sectors,
         .is_write = is_write,
+        .co = qemu_coroutine_self(),
     };
 
     qemu_co_queue_init(&req->wait_queue);
@@ -1189,6 +1191,12 @@  static void coroutine_fn wait_for_overlapping_requests(BlockDriverState *bs,
         QLIST_FOREACH(req, &bs->tracked_requests, list) {
             if (tracked_request_overlaps(req, cluster_sector_num,
                                          cluster_nb_sectors)) {
+                /* Hitting this means there was a reentrant request, for
+                 * example, a block driver issuing nested requests.  This must
+                 * never happen since it means deadlock.
+                 */
+                assert(qemu_coroutine_self() != req->co);
+
                 qemu_co_queue_wait(&req->wait_queue);
                 retry = true;
                 break;