linux-kernel - Re: [PATCH] nohz/cpuset: Make a CPU stick with do_timer() duty in the presence of nohz cpusets

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:	Fri, 30 Nov 2012 18:38:16 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Hakan Akkan <hakanakkan@...il.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] nohz/cpuset: Make a CPU stick with do_timer() duty in the
 presence of nohz cpusets

2012/11/28 Hakan Akkan <hakanakkan@...il.com>:
> +static int check_drop_timer_duty(int cpu)
> +{
> +       int curr_handler, prev_handler, new_handler;
> +       int nrepeat = -1;
> +       bool drop_recheck;
> +
> +repeat:
> +       WARN_ON_ONCE(++nrepeat > 1);
> +       drop_recheck = false;
> +       curr_handler = cpu;
> +       new_handler = TICK_DO_TIMER_NONE;
> +
> +#ifdef CONFIG_CPUSETS_NO_HZ
> +       if (atomic_read(&nr_cpus_user_nohz) > 0) {

Note atomic_read() is not SMP ordered. If another CPU does
atomic_add() or atomic_add_return(), you may not see the new value as
expected with atomic_read(). Doing atomic_add_return(0, &atomic_thing)
returns the fully ordered result.

You also need to do that to ensure full ordering against
tick_cpu_sched.user_nohz.

On the current layout we have:

(Write side)                                      (Read side)

ts->user_nohz = 1;
atomic_inc(&nr_cpus_user_nohz)

                                                      if
(atomic_read(&nr_cpus_user_nohz))
                                                            if
(per_cpu(tick_cpu_sched, curr_handler).user_nohz)
                                                                ....

If you want to make sure that you see the expected value on user_nohz
from the read side, you need to correctly order the write and read
against nr_cpus_user_nohz.

For this you can use atomic_add_return() which implies the full barrier:


(Write side)                                      (Read side)

ts->user_nohz = 1;
atomic_inc_return(&nr_cpus_user_nohz)

                                                      if
(atomic_add_return(0, &nr_cpus_user_nohz))
                                                            if
(per_cpu(tick_cpu_sched, curr_handler).user_nohz)
                                                                ....

I have much more comments on this patch, I will come back on this soon.

Thanks.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/