runtime: Hang while waiting for runtime to suspend thread in busy loop

Ran perf tests overnight on Windows 7, WOW64, 7.0 preview 2 at 1615de6c and found one busy hung in the morning.

One thread is trying to allocate an object, and GC has decided to run (fwiw the heap is not large)

0:005> kp
# ChildEBP RetAddr      
00 0829f448 75221629     ntdll!ZwWaitForSingleObject(void)+0x15 [d:\win7.obj.x86fre\minkernel\ntdll\wow6432\objfre\i386\usrstubs.asm @ 135] 
01 0829f448 75111194     KERNELBASE!WaitForSingleObjectEx(void * hHandle = 0x00000088, unsigned long dwMilliseconds = 1, int bAlertable = 0n0)+0x98 [d:\win7\minkernel\kernelbase\synch.c @ 1301] 
02 0829f460 73346fc4     kernel32!WaitForSingleObjectExImplementation(void * hHandle = 0x00000088, unsigned long dwMilliseconds = 1, int bAlertable = 0n0)+0x75 [d:\win7\base\win32\client\synch.c @ 345] 
03 (Inline) --------     coreclr!CLREventWaitHelper2(void)+0x9 [D:\a\_work\1\s\src\coreclr\vm\synch.cpp @ 372] 
04 0829f4b4 73346f7b     coreclr!CLREventWaitHelper(void * handle = <Value unavailable error>, unsigned long dwMilliseconds = <Value unavailable error>, int alertable = 0n0)+0x2d [D:\a\_work\1\s\src\coreclr\vm\synch.cpp @ 397] 
05 0829f4c4 733469c0     coreclr!CLREventBase::WaitEx(unsigned long dwMilliseconds = 1, WaitMode mode = WaitMode_None (0n0), struct PendingSync * syncState = 0x00000000)+0x32 [D:\a\_work\1\s\src\coreclr\vm\synch.cpp @ 469] 
06 0829f4dc 73335628     coreclr!CLREventBase::Wait(unsigned long dwMilliseconds = 1, int alertable = 0n0, struct PendingSync * syncState = 0x00000000)+0x1a [D:\a\_work\1\s\src\coreclr\vm\synch.cpp @ 413] 
07 0829f560 733705ac     coreclr!ThreadSuspend::SuspendRuntime(ThreadSuspend::SUSPEND_REASON reason = <Value unavailable error>)+0x54a [D:\a\_work\1\s\src\coreclr\vm\threadsuspend.cpp @ 3563] 
08 0829f60c 73370452     coreclr!ThreadSuspend::SuspendEE(ThreadSuspend::SUSPEND_REASON reason = SUSPEND_FOR_GC (0n1))+0xf9 [D:\a\_work\1\s\src\coreclr\vm\threadsuspend.cpp @ 5714] 
09 0829f634 733708cc     coreclr!GCToEEInterface::SuspendEE(SUSPEND_REASON reason = SUSPEND_FOR_GC (0n1))+0x26 [D:\a\_work\1\s\src\coreclr\vm\gcenv.ee.cpp @ 28] 
0a 0829f634 73370415     coreclr!WKS::GCHeap::GarbageCollectGeneration(unsigned int gen = 0, gc_reason reason = reason_alloc_soh (0n0))+0xa8 [D:\a\_work\1\s\src\coreclr\gc\gc.cpp @ 45101] 
0b 0829f658 732ef1b3     coreclr!WKS::gc_heap::trigger_gc_for_alloc(int gen_number = <Value unavailable error>, gc_reason gr = <Value unavailable error>, struct WKS::GCDebugSpinLock * msl = 0x73681a18, bool loh_p = false)+0x19 [D:\a\_work\1\s\src\coreclr\gc\gc.cpp @ 17075] 
0c 0829f68c 733c3bb4     coreclr!WKS::gc_heap::try_allocate_more_space(struct alloc_context * acontext = 0x0255f270, unsigned int size = 0x10, unsigned int flags = 2, int gen_number = 0n0)+0x1e4 [D:\a\_work\1\s\src\coreclr\gc\gc.cpp @ 17221] 
0d 0829f6a4 732ee473     coreclr!WKS::gc_heap::allocate_more_space(struct alloc_context * acontext = 0x0255f270, unsigned int size = 0x10, unsigned int flags = 2, int alloc_generation_number = 0n0)+0x18 [D:\a\_work\1\s\src\coreclr\gc\gc.cpp @ 17685] 
0e (Inline) --------     coreclr!WKS::gc_heap::allocate(void)+0x40 [D:\a\_work\1\s\src\coreclr\gc\gc.cpp @ 17716] 
0f 0829f6c4 732d989b     coreclr!WKS::GCHeap::Alloc(struct gc_alloc_context * context = 0x0255f270, unsigned int size = 0x10, unsigned int flags = 2)+0x53 [D:\a\_work\1\s\src\coreclr\gc\gc.cpp @ 44097] 
10 (Inline) --------     coreclr!Alloc(GC_ALLOC_FLAGS flags = <Value unavailable error>)+0x108 [D:\a\_work\1\s\src\coreclr\vm\gchelpers.cpp @ 226] 
11 0829f724 732ee14a     coreclr!AllocateObject(class MethodTable * pMT = 0x0079e774, bool fHandleCom = <Value unavailable error>)+0x175 [D:\a\_work\1\s\src\coreclr\vm\gchelpers.cpp @ 979] 
12 0829f7a8 007abb6b     coreclr!JIT_New(struct CORINFO_CLASS_STRUCT_ * typeHnd_ = 0x0079e774)+0xaa [D:\a\_work\1\s\src\coreclr\vm\jithelpers.cpp @ 2312] 
13 0829f7c0 718474f5     MicroBenchmarks!System.Collections.Concurrent.AddRemoveFromDifferentThreads`1+<>c__DisplayClass8_0[[System.Int32, System.Private.CoreLib]].<SetupConcurrentStackIteration>b__0(<Win32 error 0n318>)+0x4b

It seems to be waiting for this thread to suspend

0:006> kp
 # ChildEBP RetAddr      
00 05f5f880 718474f5     MicroBenchmarks!System.Collections.Concurrent.AddRemoveFromDifferentThreads`1+<>c__DisplayClass8_0[[System.Int32, System.Private.CoreLib]].<SetupConcurrentStackIteration>b__1(<Win32 error 0n318>)+0x43
01 05f5f890 7184b61d     System_Private_CoreLib!System.Threading.Tasks.Task.InnerInvoke(System.Threading.Tasks.Task this = <Value unavailable error>)+0x35 [/_/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs @ 2411] 

and that thread is in a busy loop trying to pop from a ConcurrentStack. the stack is empty, so I believe TryPop is returning false immediately. it’s partially inlined

                while (count < Size)
                {
                    if (stack.TryPop(out T _))
                    {
                        count++;
                    }
                }

Should we have emitted interruptible code here? Or is it by design that if you write such code you may hang at an unpredicatable moment?

Dumps shared offline.