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Date:   Sat, 2 May 2020 11:31:05 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Davidlohr Bueso <dave@stgolabs.net>
Cc:     akpm@linux-foundation.org, tglx@linutronix.de, mingo@kernel.org,
        mgorman@techsingularity.net, bp@suse.de,
        linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH] kernel/sys: do not use tasklist_lock to set/get
 scheduling priorities
Message-ID: <20200502093105.GJ3762@hirez.programming.kicks-ass.net>
References: <20200502030539.32581-1-dave@stgolabs.net>
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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.