linux-kernel - Re: [patch 20/20] cpu/hotplug: Convert hotplug locking to percpu rwsem

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]

Date:   Mon, 17 Apr 2017 08:50:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Siewior <bigeasy@...utronix.de>
Subject: Re: [patch 20/20] cpu/hotplug: Convert hotplug locking to percpu
 rwsem

On Sat, Apr 15, 2017 at 07:01:27PM +0200, Thomas Gleixner wrote:
> There are no more (known) nested calls to get_online_cpus() so it's
> possible to remove the nested call magic and convert the mutex to a
> percpu-rwsem, which speeds up get/put_online_cpus() significantly for the
> uncontended case.
> 
> The contended case (write locked for hotplug operations) is slow anyway, so
> the slightly more expensive down_write of the percpu rwsem does not matter.

So the previous thing was reader biassed and thus prone to writer
starvation. So a slightly more expensive write path doesn't matter; esp.
as its now fair and provides a guarantee it will happen, unlike the
previous one.