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Date: Mon, 18 Apr 2022 21:48:23 +0900
From: Takashi Yano <takashi.yano@nifty.ne.jp>
To: cygwin@cygwin.com
Subject: Re: Deadlock when pressing Ctrl-C at startup of a native console
 application
Message-Id: <20220418214823.a01ed743ceb15d3708ee3fd9@nifty.ne.jp>
In-Reply-To: <20220416181650.cdb31ec9c75189724bf2f498@nifty.ne.jp>
References: <355a1f23bf39b0670110a840075224a2@ispras.ru>
 <20220416181650.cdb31ec9c75189724bf2f498@nifty.ne.jp>
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Cc: Alexey Izbyshev <izbyshev@ispras.ru>
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On Sat, 16 Apr 2022 18:16:50 +0900
Takashi Yano wrote:
> On Fri, 15 Apr 2022 21:09:05 +0300
> Alexey Izbyshev wrote:
> > Hi,
> > 
> > I've discovered that pressing Ctrl-C in a Cygwin ssh session running the 
> > following command can cause a deadlock:
> > 
> > $ while python -c ''; do :; done
> > 
> > The exact native application doesn't matter here, e.g. "ping -n 1 
> > localhost >/dev/null" instead of "python -c ''" also "works".
> > 
> > The deadlock is most easily triggered when CPU is fully loaded. It takes 
> > no more than 20 tries on my machine to hit it (and when I first saw it, 
> > it was on the first try).
> > 
> > When the deadlock is triggered, there are conhost.exe and 
> > cygwin-console-helper.exe processes, but there is no python.
> > 
> > The exact process tree varies depending on Cygwin version. With the 
> > vanilla 3.3.4 (the root is bash spawned by sshd):
> > 
> > bash---bash-+-conhost.exe
> >              `-cygwin-console-helper.exe
> > 
> > With 20220301 snapshot 
> > (https://cygwin.com/snapshots/x86/cygwin1-20220301.dll.xz), I get one of 
> > the two following trees with an extra conhost.exe process:
> > 
> > bash---bash-+-conhost.exe
> >              |-conhost.exe
> >              `-cygwin-console-helper.exe
> > 
> > or
> > 
> > bash---bash-+-conhost.exe
> >       |      `-cygwin-console-helper.exe
> >       `-conhost.exe
> > 
> > I was testing with 32-bit Cygwin on x64 Windows 10 21H2.
> 
> Thanks for the report. I could reproduce the issue.
> I also found the current git master does not have this issue,
> while cygwin-3_3-branch does.
> 
> Now I am identifying which patch solved the issue.
> One is obviously:
> https://cygwin.com/pipermail/cygwin-patches/2022q2/011870.html
> however, this was applied also for cygwin-3_3-branch. Therefore,
> another patch also should affect this issue.

I identified the cause and applied a workaround patch.
The countermeasure version is available at:
https://tyan0.yr32.net/cygwin/x86/test/cygwin1-20220418.dll.xz
https://tyan0.yr32.net/cygwin/x86_64/test/cygwin1-20220418.dll.xz

Could you please test?

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

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