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Commit a556d071 authored by ramindani's avatar ramindani Committed by Ram Indani
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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
parent 900cec50

services/surfaceflinger/Scheduler/Scheduler.cpp

+7 −2
Original line number Diff line number Diff line
@@ -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.

        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();