CentOS virt - Aug 2017 - Status of reverted Linux patch "tty: Fix ldisc crash on reopened tty", Linux 4.9 kernel frequent crashes

Hello everyone,

Recently Nathan March reported on centos-virt list he's getting frequent
Linux kernel crashes with Linux 4.9 LTS kernel because of the missing patch
"tty: Fix ldisc crash on reopened tty".

The patch was already merged upstream here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=71472fa9c52b1da27663c275d416d8654b905f05

but then reverted here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=896d81fefe5d1919537db2c2150ab6384e4a6610

Nathan confirmed if he applies the patch from
71472fa9c52b1da27663c275d416d8654b905f05 to his Linux 4.9 LTS kernel the
bug/problem goes away, so the patch (or similar fix) is still needed, at least
for 4.9 LTS kernel.


Mikulas reported he's able to trigger the same crash on Linux 4.10:
https://www.spinics.net/lists/kernel/msg2440637.html
https://lists.gt.net/linux/kernel/2664604?search_string=ldisc%20reopened;#2664604

Michael Neuling reported he's able to trigger the bug on PowerPC:
https://lkml.org/lkml/2017/3/10/1582


So now the question is.. is anyone currently working on getting this patch fixed
and applied upstream? I think one of the problems earlier was being able to
reliable reproduce the crash.. Nathan says he's able to reproduce it many
times per week on his environment on x86_64.


Thanks a lot,

-- Pasi

On 08/30/2017 03:10 PM, Pasi K?rkk?inen wrote:
> Hello everyone,
> 
> Recently Nathan March reported on centos-virt list he's getting
frequent Linux kernel crashes with Linux 4.9 LTS kernel because of the missing
patch "tty: Fix ldisc crash on reopened tty".
> 
> The patch was already merged upstream here:
>
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=71472fa9c52b1da27663c275d416d8654b905f05
> 
> but then reverted here:
>
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=896d81fefe5d1919537db2c2150ab6384e4a6610
> 
> Nathan confirmed if he applies the patch from
71472fa9c52b1da27663c275d416d8654b905f05 to his Linux 4.9 LTS kernel the
bug/problem goes away, so the patch (or similar fix) is still needed, at least
for 4.9 LTS kernel.
> 
> 
> Mikulas reported he's able to trigger the same crash on Linux 4.10:
> https://www.spinics.net/lists/kernel/msg2440637.html
>
https://lists.gt.net/linux/kernel/2664604?search_string=ldisc%20reopened;#2664604
> 
> Michael Neuling reported he's able to trigger the bug on PowerPC:
> https://lkml.org/lkml/2017/3/10/1582
> 
> 
> So now the question is.. is anyone currently working on getting this patch
fixed and applied upstream? I think one of the problems earlier was being able
to reliable reproduce the crash.. Nathan says he's able to reproduce it many
times per week on his environment on x86_64.
I looked briefly at the patch and related discussion on the kernel
mailing lists and it seemed to be reverted not due to any problems it
caused with kernel behavior but rather due to concerns about
insufficient review before it was committed and possible merge conflicts.

The issue is the problem doesn't appear to have been discussed any
further on the kernel mailing lists since April, and I'm not sure why.
My inclination would be to start back up discussion upstream and try to
get clarification as to why the patch has remained reverted and there's
been no effort to bring it back into the kernel, rather than assume the
patch is safe to use.  I doubt anyone else but people experiencing the
issue have it up on their radar.

The 4.9 virt kernel does have some patches currently that haven't (yet)
been accepted upstream, so it's definitely an option to do that here.
As far as I know, nothing really gets pushed back upstream.  Most often
the patches are just plucked from upstream mailing lists before they get
merged into an official upstream release.

-- 
Kevin Stange
Chief Technology Officer
Steadfast | Managed Infrastructure, Datacenter and Cloud Services
800 S Wells, Suite 190 | Chicago, IL 60607
312.602.2689 X203 | Fax: 312.602.2688
kevin at steadfast.net | www.steadfast.net

On 08/30/2017 03:10 PM, Pasi K?rkk?inen wrote:
> Hello everyone,
> 
> Recently Nathan March reported on centos-virt list he's getting
frequent Linux kernel crashes with Linux 4.9 LTS kernel because of the missing
patch "tty: Fix ldisc crash on reopened tty".
> 
> The patch was already merged upstream here:
>
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=71472fa9c52b1da27663c275d416d8654b905f05
> 
> but then reverted here:
>
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=896d81fefe5d1919537db2c2150ab6384e4a6610
> 
> Nathan confirmed if he applies the patch from
71472fa9c52b1da27663c275d416d8654b905f05 to his Linux 4.9 LTS kernel the
bug/problem goes away, so the patch (or similar fix) is still needed, at least
for 4.9 LTS kernel.
> 
> 
> Mikulas reported he's able to trigger the same crash on Linux 4.10:
> https://www.spinics.net/lists/kernel/msg2440637.html
>
https://lists.gt.net/linux/kernel/2664604?search_string=ldisc%20reopened;#2664604
> 
> Michael Neuling reported he's able to trigger the bug on PowerPC:
> https://lkml.org/lkml/2017/3/10/1582
> 
> 
> So now the question is.. is anyone currently working on getting this patch
fixed and applied upstream? I think one of the problems earlier was being able
to reliable reproduce the crash.. Nathan says he's able to reproduce it many
times per week on his environment on x86_64.
> 
I have no issues rolling this patch in , while we wait on upstream, if
it makes our tree more stable.


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On Sat, Sep 02, 2017 at 08:12:53AM -0500, Johnny Hughes
wrote:
> On 08/30/2017 03:10 PM, Pasi K?rkk?inen wrote:
> > Hello everyone,
> > 
> > Recently Nathan March reported on centos-virt list he's getting
frequent Linux kernel crashes with Linux 4.9 LTS kernel because of the missing
patch "tty: Fix ldisc crash on reopened tty".
> > 
> > The patch was already merged upstream here:
> >
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=71472fa9c52b1da27663c275d416d8654b905f05
> > 
> > but then reverted here:
> >
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=896d81fefe5d1919537db2c2150ab6384e4a6610
> > 
> > Nathan confirmed if he applies the patch from
71472fa9c52b1da27663c275d416d8654b905f05 to his Linux 4.9 LTS kernel the
bug/problem goes away, so the patch (or similar fix) is still needed, at least
for 4.9 LTS kernel.
> > 
> > 
> > Mikulas reported he's able to trigger the same crash on Linux
4.10:
> > https://www.spinics.net/lists/kernel/msg2440637.html
> >
https://lists.gt.net/linux/kernel/2664604?search_string=ldisc%20reopened;#2664604
> > 
> > Michael Neuling reported he's able to trigger the bug on PowerPC:
> > https://lkml.org/lkml/2017/3/10/1582
> > 
> > 
> > So now the question is.. is anyone currently working on getting this
patch fixed and applied upstream? I think one of the problems earlier was being
able to reliable reproduce the crash.. Nathan says he's able to reproduce it
many times per week on his environment on x86_64.
> > 
> 
> I have no issues rolling this patch in , while we wait on upstream, if
> it makes our tree more stable.
> 
I think we should do that.. What do others think?


Thanks,

-- Pasi