linux-kernel - Re: [GIT PULL] signal/exit/ptrace changes for v5.17

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Date:   Mon, 17 Jan 2022 17:44:08 +0200
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Gladkov <legion@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [GIT PULL] signal/exit/ptrace changes for v5.17

On Mon, Jan 17, 2022 at 5:32 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> I would like to have a version of pipe_write that sleeps in
> TASK_KILLABLE.

That would actually be horrible for another reason - now it would
count towards the load average. That's another difference between
interruptible waits and non-interruptible ones.

Admittedly it's an entirely arbitrary one, but it's part of the whole
semantic difference between TASK_INTERRUPTIBLE and
TASK_UNINTERRUPTIBLE.

You can play with TASK_NOLOAD of course, so it's something that can be
worked around, but it gets a bit ugly.

>  I want the I/O wake-ups and I want the SIGKILL wake ups
> but I don't want any other wake-ups.  Unfortunately the I/O wake-ups in
> the pipe code are sent with wake_up_interruptible.  So a task sleeping
> in TASK_KILLABLE won't get them.

Yeah. The code *could* use the non-interruptible 'wake_up()', and
everything should work - because waking things up too much doesn't
change semantics, it's just a slight pessimization. Plus the whole
"nested waitqueues" isn't actually any remotely normal case, so it
doesn't really matter for performance either.

But I really think it's wrong.

You're trying to work around a problem the wrong way around. If a task
is dead, and is dumping core, then signals just shouldn't matter in
the first place, and thus the whole "TASK_INTERRUPTIBLE vs
TASK_UNINTERRUPTIBLE" really shouldn't be an issue. The fact that it
is an issue means there's something wrong in signaling, not in the
pipe code.

So I really think that's where the fix should be - on the signal delivery side.

             Linus