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>

0 files changed, 0 insertions, 0 deletions