linux-kernel - Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates

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: <20210728184651.GJ4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 28 Jul 2021 11:46:51 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        rcu <rcu@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>, Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>
Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates

On Wed, Jul 28, 2021 at 02:12:05PM -0400, Mathieu Desnoyers wrote:
> ----- On Jul 28, 2021, at 1:58 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:
> 
> > On Wed, Jul 28, 2021 at 10:37 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> >>
> >> +/*
> >> + * Increment the current CPU's rcu_data structure's ->dynticks field
> >> + * with ordering.  Return the new value.
> >> + */
> >> +static noinstr unsigned long rcu_dynticks_inc(int incby)
> >> +{
> >> +       return arch_atomic_add_return(incby, this_cpu_ptr(&rcu_data.dynticks));
> >> +}
> > 
> > Maybe inline?
> > 
> > But more I reacted to how we sadly don't have percpu atomics. They'd
> > be fairly easy to add on x86, but I guess it's not a huge deal.
> 
> Are the percpu atomics you have in mind different from what is found in
> Documentation/core-api/this_cpu_ops.rst ?
> 
> Namely this_cpu_add_return(pcp, val) in this case.
> 
> I must be missing something subtle because AFAIU those are already
> available. Those per-cpu atomics don't provide any memory ordering
> though, which may be why those are not used here.

Good point, but this code does indeed need the ordering.  It also must
support the occasional off-CPU access.

							Thanx, Paul