Deadlock when pressing Ctrl-C at startup of a native console application

Alexey Izbyshev izbyshev@ispras.ru
Mon Apr 18 15:07:16 GMT 2022


On 2022-04-18 15:48, Takashi Yano wrote:
> 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?

I've tested the 32-bit version, and Ctrl-C in my original test works as 
expected (no deadlock or other issues after multiple tries). Thank you 
for the fix!

Alexey


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