linux-kernel - Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1

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Message-ID: <AM6PR03MB5170CCB8D9D41904066DAFD5E4AA0@AM6PR03MB5170.eurprd03.prod.outlook.com>
Date:   Thu, 30 Apr 2020 15:39:23 +0200
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Gladkov <gladkov.alexey@...il.com>
Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1

On 4/30/20 4:16 AM, Linus Torvalds wrote:
> On Wed, Apr 29, 2020 at 5:00 PM Jann Horn <jannh@...gle.com> wrote:
>>
>> Wouldn't you end up livelocked in the scenario that currently deadlocks?
> 
> The test case that we already know is broken, and any fix will have to
> change anyway?
> 

The purpose of the test case was only to test the behaviour of my
later patch.  The test case _must_ be adjusted to the follow-up
patch, I have no problem with that.  Anybody may change the test case
when we know how to fix the API.  I did just not anticipate that Eric
would only apply 14 of 16 patches = 87.5% of the patch series. Now that
causes some tension, but it is not really a problem IMHO.

> Let's just say that I don't care in the least.
> 
> But Bernd's patch as-is breaks a test-case that currently *works*,
> namely something as simple as
> 
>   echo xyz > /proc/<pid>/attr/something
> 

Excuse me, but what in my /proc folder there is no attr/something
is there a procfs equivalent of pthread_attach ?

What exactly is "attr/something" ?

> and honestly, breaking something that _works_ and may be used in
> reality, in orderf to make a known buggy user testcase work?
> 
> Because no, "write()" returning -EAGAIN isn't ok.
> 

write can return -EAGAIN if the file is non-blocking, it is
never the case for a disk file, but for a NFS that is not at all
clear, depends on a mount option, and once I had a deadlock in
one of my test systems after OOM-stress, but I never was able
to reproduce, the umount deadlocked, then I was not able to
reboot, could be an alpha-particle of course, who knows...

Hmmm.. maybe a stupid idea:

We could keep the old deadlock-capable API,
and add a new _flag_ somewhere to the PTHREAD_ATTACH call,
that _enables_ the non-blocking behavior, how about that.

Thanks
Bernd.

>             Linus
>