[PATCH hwc v2 02/18] drm_hwcomposer: vsyncworker: Fix deadlock on exit path

[Alexandru Gheorghe]

vsyncworker::Routine assumes that when -EINTR is returned by
WaitForSignalOrExitLocked the lock as been released, which is not
true, so it hangs if a vsyncworker is never enabled and Exit is
called.

There are two code paths in WaitForSignalOrExitLocked that return
-EINTR, one releases the lock the other doesn't.
Looking at the clients of WaitForSignalOrExitLocked all assume the lock
is still held, except vsyncworker::Routine.
So, the proper fix needs two changes:
- Make WaitForSignalOrExitLocked consistent and always hold the lock
  when exiting.
- Release lock in vsynworker::Routine on all code paths.

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe at arm.com>
---
 vsyncworker.cpp | 1 +
 worker.cpp      | 6 +++---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/vsyncworker.cpp b/vsyncworker.cpp
index 3bfe4be..7c0c741 100644
--- a/vsyncworker.cpp
+++ b/vsyncworker.cpp
@@ -120,6 +120,7 @@ void VSyncWorker::Routine() {
   if (!enabled_) {
     ret = WaitForSignalOrExitLocked();
     if (ret == -EINTR) {
+      Unlock();
       return;
     }
   }
diff --git a/worker.cpp b/worker.cpp
index da6c580..5b351e0 100644
--- a/worker.cpp
+++ b/worker.cpp
@@ -66,13 +66,13 @@ int Worker::WaitForSignalOrExitLocked(int64_t max_nanoseconds) {
     ret = -ETIMEDOUT;
   }
 
+  // release leaves mutex locked when going out of scope
+  lk.release();
+
   // exit takes precedence on timeout
   if (should_exit())
     ret = -EINTR;
 
-  // release leaves mutex locked when going out of scope
-  lk.release();
-
   return ret;
 }
 
-- 
2.7.4