On Thu, 14 Apr 2022 02:17:38 +0300 Alexey Izbyshev wrote: > On 2022-04-13 19:48, Alexey Izbyshev wrote: > > On 2022-04-11 13:10, Alexey Izbyshev wrote: > > What's probably not normal is the behavior of the hanging conhost.exe. > > I've compared the points where conhost.exe is blocked, and all but one > > threads in the model case are doing the same things as in the hanging > > case, but the remaining thread is blocked in > > ReadFile("\Device\NamedPipe\") (i.e. the read end of "hWritePipe" of > > pcon) instead of trying to enter a critical section like thread 1 > > above. So now I'm starting to doubt that it's a cygwin bug and not > > some conhost.exe bug. > > > > I'll try to poke around the hanging conhost.exe some more, and also > > may be will try to create a faster reproducer. > > > I've studied conhost.exe hang, and it indeed looks like it's buggy. > > TLDR: https://github.com/microsoft/terminal/pull/12181 > > The full story: > > I dumped conhost.exe, opened the dump in windbg and looked at the stack > trace of the hanging thread: > > ntdll!NtWaitForAlertByThreadId+0x14 > ntdll!RtlpWaitOnAddressWithTimeout+0x81 > ntdll!RtlpWaitOnAddress+0xae > ntdll!RtlpWaitOnCriticalSection+0xfd > ntdll!RtlpEnterCriticalSectionContended+0x1c4 > ntdll!RtlEnterCriticalSection+0x42 > conhost!Microsoft::Console::Render::Renderer::_PaintFrameForEngine+0x54 > conhost!Microsoft::Console::Render::Renderer::TriggerTeardown+0x19e60 > conhost!Microsoft::Console::Interactivity::ServiceLocator::RundownAndExit+0x21 > conhost!Microsoft::Console::PtySignalInputThread::_GetData+0x65 > conhost!Microsoft::Console::PtySignalInputThread::_InputThread+0x25 > kernel32!BaseThreadInitThunk+0x14 > ntdll!RtlUserThreadStart+0x21 > > By looking at assembly, I've found that it hangs *after* ReadFile() on > the pipe completes, so the problem is definitely not a leak of > hWritePipe in bash.exe or elsewhere. > > Using the function names, I've found this issue: > https://github.com/microsoft/terminal/issues/1810. > > This is a different one, but the discussion and the patch shows that > synchronization on startup/shutdown is a disaster. > > Then I looked at the code and identified that hang happens while > attempting to lock the console at [1]. After studying how this lock is > used in other parts of the code, I noticed that > PtySignalInputThread::_Shutdown() (which is further up in the call stack > of the hanging function) uses ProcessCtrlEvents() incorrectly, because > the latter unconditionally unlocks the console, but the lock is never > taken by this thread at this point. Then I looked at a more recent > version of the code and discovered the patch to _Shutdown() which I > referenced above. > > I've also verified that assembly of _Shutdown() (which is inlined into > PtySignalInputThread::_GetData()) corresponds to the unpatched version > (i.e. without LockConsole() call): > > call conhost!CloseConsoleProcessState (00007ff6`22e7013c) > call conhost!ProcessCtrlEvents (00007ff6`22e262a0) > mov ecx,6Dh > call > conhost!Microsoft::Console::Interactivity::ServiceLocator::RundownAndExit > (00007ff6`22e3c730) > > I'm not sure why this bug is not triggered more frequently, but one > possible reason, as indicated by comment [2], is that the bad path is > only taken if there are live clients after ClosePseudoConsole() is > called, which is probably rare. > > A potential workaround on Cygwin side would be to ensure that the > pseudoconsole doesn't have clients before calling ClosePseudoConsole(), > but I don't know whether it's possible.
I am not sure yet what is essential, but the current code closes pseudo console only if there is no other process which is attaching to the pseudo console. I wonder why javac.exe is remaining as zombie. The parent bash.exe calls ColosePseudoConsole() when child non-cygwin app is terminated, i.e., after WaitForSingleObject() for child process handle returns. https://www.cygwin.com/git/?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/spawn.cc;h=81dba5a941e919ea2514013069aef22c6fad8004;hb=7ac0767053e278f0ce9811bf6f77278bd2f49c20#l1009 What does the "zombie" mean? Is it listed in the process list of ProcessHacker? I still suspect that the zombie javac.exe holds the hWritePipe handle leaked from parent bash.exe. > [1] > https://github.com/microsoft/terminal/blob/9b92986b49bed8cc41fde4d6ef080921c41e6d9e/src/renderer/base/renderer.cpp#L75 > > [2] > https://github.com/microsoft/terminal/blob/9b92986b49bed8cc41fde4d6ef080921c41e6d9e/src/host/PtySignalInputThread.cpp#L205 -- Takashi Yano <takashi.y...@nifty.ne.jp> -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple