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

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

[Alexey Izbyshev]

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,
Alexey

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

[Takashi Yano]

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.

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

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

[Takashi Yano]

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>

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

[Alexey Izbyshev]

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