linux-kernel - Re: [PATCH] ARM: Don't ever downscale loops_per

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: <20140509091824.GL3693@n2100.arm.linux.org.uk>
Date:	Fri, 9 May 2014 10:18:24 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Doug Anderson <dianders@...omium.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Will Deacon <will.deacon@....com>,
	John Stultz <john.stultz@...aro.org>,
	David Riley <davidriley@...omium.org>,
	"olof@...om.net" <olof@...om.net>,
	Sonny Rao <sonnyrao@...omium.org>,
	Richard Zhao <richard.zhao@...aro.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Shawn Guo <shawn.guo@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Stephen Warren <swarren@...dia.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP
	systems#

On Thu, May 08, 2014 at 09:37:15PM -0400, Nicolas Pitre wrote:
> On Thu, 8 May 2014, Russell King - ARM Linux wrote:
> 
> > If you're in a preempt or SMP environment, provide a timer for udelay().
> > IF you're in an environment with IRQs which can take a long time, use
> > a timer for udelay().  If you're in an environment where the CPU clock
> > can change unexpectedly, use a timer for udelay().
> 
> Longer delays are normally not a problem.  If they are, then simply 
> disabling IRQs may solve it if absolutely required.  With much shorter 
> delays than expected this is another story.
> 
> What about the following:
> 
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 7c4fada440..10030cc5a0 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -682,6 +682,15 @@ static int cpufreq_callback(struct notifier_block *nb,
>  			cpufreq_scale(per_cpu(l_p_j_ref, cpu),
>  					per_cpu(l_p_j_ref_freq, cpu),
>  					freq->new);
> +		/*
> +		 * Another CPU might have called udelay() just before LPJ
> +		 * and a shared CPU clock is increased.  That other CPU still
> +		 * looping on the old LPJ value would return significantly
> +		 * sooner than expected.  The actual fix is to provide a
> +		 * timer based udelay() implementation instead.
> +		 */
> +		if (freq->old < freq->new)
> +			pr_warn_once("*** udelay() on SMP is racy and may be much shorter than expected ***\n");
>  	}
>  	return NOTIFY_OK;
>  }

No, because you're assuming this is just a SMP problem.  What about
preempt, where you could preempt away from a udelay loop to change
the CPU frequency, and then back again, possibly resulting in the
CPU clock rate increasing and maybe a shorter delay if the switch
from-change-clock-and-back is fast enough?  Remember that udelay()
can be used for up to 2ms delays.

-- 
FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly
improving, and getting towards what was expected from it.
--
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/