linux-kernel - Re: [PATCH] kernel/sys: do not use tasklist

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Message-ID: <20200502093105.GJ3762@hirez.programming.kicks-ass.net>
Date:   Sat, 2 May 2020 11:31:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     akpm@...ux-foundation.org, tglx@...utronix.de, mingo@...nel.org,
        mgorman@...hsingularity.net, bp@...e.de,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] kernel/sys: do not use tasklist_lock to set/get
 scheduling priorities

On Fri, May 01, 2020 at 08:05:39PM -0700, Davidlohr Bueso wrote:
> For both setpriority(2) and getpriority(2) there's really no need
> to be taking the tasklist_lock at all - for which both share it
> for the entirety of the syscall. The tasklist_lock does not protect
> reading/writing the p->static_prio and task lookups are already rcu
> safe, providing a stable pointer.

RCU-safe, as in, it will not crash.. However, without tasklist_lock the
thread iterations (for PRIO_PGRP/PRIO_USER) now race against fork().

That is a user observable change in behaviour.

Do we care about it? No idea, and your Changelog also doesn't provide
clue.