linux-kernel - Re: [PATCH v2] exit: perform randomness and pid work without tasklist

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Message-ID: <CAGudoHHd3cE6+nyZY3wi5Xw5++uKiypR3qK_3=2XcGNocE4Vyw@mail.gmail.com>
Date: Fri, 31 Jan 2025 23:31:01 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: brauner@...nel.org, oleg@...hat.com, akpm@...ux-foundation.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] exit: perform randomness and pid work without tasklist_lock

On Fri, Jan 31, 2025 at 9:56 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Moving proc_flush_pid inside of tasklist_lock is a bad idea.

The patch does not make such a change though.

The call is still performed without the lock, but it also dodges the
additional refcount dance (and notably eliminates an atomic from an
area protected by tasklist_lock).

>
> It is wrong that attach_pid/detach_pid can be performed without the
> tasklist_lock.  There are reasonable guarantees provided by the posix
> standard that the set of processes sent a signal is the set of
> processes at a point in time.  The tasklist_lock is how we provide
> those guarantees currently.
>

I don't see anything calling these without the lock and neither my
patch nor a follow up about pids suggest anything of the sort.

> There are two more layers to pids.  The pid number allocation of
> alloc_pid/free_pid, and the struct pid layer maintained by get_pid,
> put_pid.  Those two layers don't need the tasklist_lock.
>
>
> It is safe to move free_pid out of tasklist_lock.  I am not certain
> how sane it is.
>

Where is the sanity problem here? AFAICS this just delays some wakeups
in the worst case.

Regardless, looks like I have enough to send a v2 for further commentary.

-- 
Mateusz Guzik <mjguzik gmail.com>