linux-kernel - Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use rcu_read_lock_sched

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87359mdeg3.ffs@tglx>
Date:   Sun, 11 Dec 2022 00:30:36 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Karol Herbst <karolherbst@...il.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>
Subject: Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use
 rcu_read_lock_sched_notrace()

On Sat, Dec 10 2022 at 13:34, Steven Rostedt wrote:
> On Sat, 10 Dec 2022 09:47:53 -0800 "Paul E. McKenney" <paulmck@...nel.org> wrote:
>> This does mess with preempt_count() redundantly, but the overhead from
>> that should be way down in the noise.
>
> I was going to remove it, but then I realized that it would be a functional
> change, as from the comment above, it uses "preempt_enable_no_resched(),
> which there is not a rcu_read_unlock_sched() variant.

preempt_enable_no_resched() in this context is simply garbage.

preempt_enable_no_resched() tries to avoid the overhead of checking
whether rescheduling is due after decrementing preempt_count() because
the code which it this claims to know that it is _not_ the outermost one
which brings preempt count back to preemtible state.

I concede that there are hot paths which actually can benefit, but this
code has exactly _ZERO_ benefit from that. Taking that tracing exception
and handling it is orders of magnitudes more expensive than a regular
preempt_enable().

So just get rid of it and don't proliferate cargo cult programming.

Thanks,

        tglx