features/qemu-block: simplify coroutine model to use single synctask, ucontext

The current coroutine model, mapping synctasks 1-1 with qemu internal
Coroutines, has some unresolved raciness issues. This problem usually
manifests as lifecycle mismatches between top-level (gluster created)
synctasks and the subsequently created internal coroutines from that
context. Qemu's internal queueing (and locking) can cause situations
where the top-level synctask is destroyed before the internal scheduler
has released references to memory, leading to use after free crashes
and asserts.

Simplify the coroutine model to use a single synctask as a coroutine
processor and rely on the existing native ucontext coroutine
implementation. The syncenv thread is donated to qemu and ensures a
single top-level coroutine is processed at a time. Qemu now has
complete control over coroutine scheduling.

BUG: 986775
Change-Id: I38223479a608d80353128e390f243933fc946fd6
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/6110
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/xlators/features/qemu-block/src/qemu-block.c b/xlators/features/qemu-block/src/qemu-block.c
index 0edb7b9493d..48bbf314048 100644
--- a/xlators/features/qemu-block/src/qemu-block.c
+++ b/xlators/features/qemu-block/src/qemu-block.c
@@ -204,6 +204,7 @@ qb_local_init (call_frame_t *frame)
 	qb_local = GF_CALLOC (1, sizeof (*qb_local), gf_qb_mt_qb_local_t);
 	if (!qb_local)
 		return -1;
+	INIT_LIST_HEAD(&qb_local->list);
 
 	qb_local->frame = frame;
 	frame->local = qb_local;