Hi there,
I am also having same crash/waiting on thread issue with 4.21 on latest OS X build 10.10 (14A283o). I’ve attach sample of process here also.
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Crash/Deadlock when running 4.21(and 4.3 Preview) on latest OSX 10.10 Yosemite (Developer Preview 3)
I can confirm that 4.21 does not run on latest version of OSX on another separate iMac also. That means I have reproduced this on a different configuration.
This is totally random. I now just continually click launch button after each failed hung load until editor finally comes up.
Hey ,
I don’t believe we’ve gotten to testing in 10.10 just yet, but I will check with developers. Just so I can give them as much information as they may need, would you mind telling us your system specs for both machines you have tested this on? And once you are able to get editor to open, does it remain open and continue to function as expected? Thanks!
Hey , I’ll get you specs shortly. Once editor is actually up, it is fine. Any subsequent startup of an editor instance i.e Launcher->Editor(Project Selector)->Editor(Actual Project) is a dice roll as to whether it will lock up on a mutex.
Also , this is a problem on 4.3 Preview also. I’m getting feeling that Apple has stuffed something up at kernel level and you’re hitting it. Looks like a failure to release a mutex 
Just to let you know , we have now confirmed that we are getting this on two other computers our group is using. Both are 10.10 preview 3 machines. Others are not, and these don’t have problem.
Mac 1&2 Specs:
Model Name: iMac
Model Identifier: iMac13,2
Processor Name: Intel Core i7
Processor Speed: 3.4 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 8 MB
Memory: 24 GB
Boot ROM Version: IM131.010A.B05
SMC Version (system): 2.11f14
NVIDIA GeForce GTX 680MX 2048 MB
Mac 3:
Model Name:iMac
Model Identifier:iMac12,2
Processor Name:Intel Core i7
Processor Speed:3.4 GHz
Number of Processors:1
Total Number of Cores:4
L2 Cache (per Core):256 KB
L3 Cache:8 MB
Memory:24 GB
Boot ROM Version:IM121.0047.B1E
SMC Version (system):1.72f2
AMD Radeon HD 6970M 2048 MB
On further inspection. execution is stuck in a do while loop at line 678 in RenderingThread.cpp:
It never exits loop because bDone is always false:
// Check rendering thread health needs to be called from time to
// time in order to pump messages, otherwise RHI may block
// on vsync causing a deadlock. Also we should make sure
// rendering thread hasn't crashed :)
bool bDone;
uint32 WaitTime = FMath::Clamp<uint32>(GTimeToBlockOnRenderFence, 0, 33);
do
{
CheckRenderingThreadHealth();
if (bEmptyGameThreadTasks)
{
// process gamehtread tasks if there are any
FTaskGraphInterface::Get().ProcessThreadUntilIdle(ENamedThreads::GameThread);
}
bDone = Event->Wait(WaitTime);
}
while (!bDone);
Just noticed that in line 330 of GenericPlatformProcess.cpp there is this while condition:
} while ((!bRetVal) && (WaitTime != 0));
In case of deadlock, bRetVal == false BUT WaitTime is always 0 
Hmmm. This means that:
FrameEndSync.Sync( CVarAllowOneFrameThreadLag->GetValueOnGameThread() != 0 );
is getting stuck at line 2151 of LaunchEngineLoop.cpp(FStats::AdvanceFrame)
Hey ,
Thanks for all information you’ve provided. Currently we are not supporting 10.10 and have not been testing for it yet. We expect Apple to continue releasing new builds of 10.10 before they release this fall, so it is possible this issue may change or new issues will arise by then. We will make sure we have full support by release, but for now if you need your project to run without issue we recommend sticking with OSX 10.9.3.
Please feel free to update us with additional issues if you continue working in 10.10, and we will work to fix them before it is released. Thanks for your report!
Hi . I’m well aware of that fact 10.10 is prerelease. point of report was to make sure that you guys were aware of current state of runnability. Note that preview builds are now open to both non-developers and developers like myself. It would probably be a good idea to leave this unresolved, until it is actually resolved, for visibility sake.
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Yes, and we’re grateful for feedback you’ve provided. Please continue to do so if you run into other problems while using 10.10, and we will do our best to make sure it is addressed before official release. I just needed to explain why we won’t be able to fix it immediately, and hope that you and any other users who switch to 10.10 early understand.
Our resolved status happens automatically when we (staff) respond to a post, and sometimes I forget to mark it back to unresolved, so I apologize for that. In this case, however, post will likely be marked resolved before long because answer is simply that we do not currently support 10.10 and will not do so until release. Please note that this does not mean we are not tracking post and I will of course make sure we continue to report any notes you make here. Thanks!
Oh sorry ! I didn’t mean to make sound like that! I totally meant that as “I wan to make sure it doesn’t get lost in my resolved pile of stuff”. I totally know you guys look at stuff! Sorry so sounding like an ■■■ 
Looks like 10.10 preview 4 released today solves problem
Excellent! Thanks for looking out for this! hugs