From a556d071c1731df6103c8eb86a52503b962c33bc Mon Sep 17 00:00:00 2001
From: ramindani <ramindani@google.com>
Date: Tue, 14 Jun 2022 23:25:11 +0000
Subject: SF: Fix deadlock while stopping idle timer

scope_lock `mRefreshRateConfigsLock` is acquired
during `Scheduler::setRefreshRateConfigs` to
stop & clear the existing idle timer callback.
And in the meantime, when the code in
`OneShotTimer::loop` encounters the timeout
and then timeoutcallback is requested from
OneShotTimer::loop, this callback in
`Scheduler::kernelIdleTimerCallback`
will try to acquire the lock on
`mRefreshRateConfigsLock`.
The OneShotTimer::stop which is called to reset
the idle timer from the
`Scheduler::setRefreshRateConfigs` now join on
the OneShotTimer::loop thread that is now blocked
waiting on the `mRefreshRateConfigsLock`. So this
call never finishes and deadlock happens.
When we use the fakeGuard to lock `mRefreshRateConfigsLock`
we fake the lock and stop the idle timer. This is okay to do
as we only update mRefreshRateConfig on the main thread.

Test: atest libsurfaceflinger_unittest
go/wm-smoke
BUG: 233423207

Change-Id: I146325193b57d81bc3423426355791ca0ec962f5
---
 services/surfaceflinger/Scheduler/Scheduler.cpp | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/services/surfaceflinger/Scheduler/Scheduler.cpp b/services/surfaceflinger/Scheduler/Scheduler.cpp
index 37f0fec425..c9840c7ac8 100644
--- a/services/surfaceflinger/Scheduler/Scheduler.cpp
+++ b/services/surfaceflinger/Scheduler/Scheduler.cpp
@@ -25,6 +25,7 @@
 #include <android/hardware/configstore/1.0/ISurfaceFlingerConfigs.h>
 #include <android/hardware/configstore/1.1/ISurfaceFlingerConfigs.h>
 #include <configstore/Utils.h>
+#include <ftl/fake_guard.h>
 #include <gui/WindowInfo.h>
 #include <system/window.h>
 #include <ui/DisplayStatInfo.h>
@@ -94,9 +95,13 @@ void Scheduler::startTimers() {
 }
 
 void Scheduler::setRefreshRateConfigs(std::shared_ptr<RefreshRateConfigs> configs) {
+    // The current RefreshRateConfigs instance may outlive this call, so unbind its idle timer.
     {
-        // The current RefreshRateConfigs instance may outlive this call, so unbind its idle timer.
-        std::scoped_lock lock(mRefreshRateConfigsLock);
+        // mRefreshRateConfigsLock is not locked here to avoid the deadlock
+        // as the callback can attempt to acquire the lock before stopIdleTimer can finish
+        // the execution. It's safe to FakeGuard as main thread is the only thread that
+        // writes to the mRefreshRateConfigs.
+        ftl::FakeGuard guard(mRefreshRateConfigsLock);
         if (mRefreshRateConfigs) {
             mRefreshRateConfigs->stopIdleTimer();
             mRefreshRateConfigs->clearIdleTimerCallbacks();
-- 
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