linux-kernel - Re: [GIT PULL] RCU changes for v5.10

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Message-ID: <CAHk-=whE1rajA5Kzqey802zwv-82yrK5qc=nR3xRo5f38t-K8A@mail.gmail.com>
Date:   Sun, 18 Oct 2020 14:39:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] RCU changes for v5.10

On Mon, Oct 12, 2020 at 7:14 AM Ingo Molnar <mingo@...nel.org> wrote:
>
> Please pull the latest core/rcu git tree from:
>
>    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core-rcu-2020-10-12

I've pulled everything but that last merge and the PREEMPT_COUNT stuff
that came with it.

When Paul asked whether it was ok for RCU to use preempt_count() and I
answered in the affirmative, I didn't mean it in the sense of "RCU
wants to force it on everybody else too".

I'm pretty convinced that the proper fix is to simply make sure that
rcu_free() and friends aren't run under any raw spinlocks. So even if
the cost of preempt-count isn't that noticeable, there just isn't a
reason for RCU to say "screw everybody else, I want this" when there
are other alternatives.

            Linus