linux-kernel - Re: [PATCH 6/7] clocksource: Add Pistachio clocksource-only driver

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: <555E55F4.2060500@imgtec.com>
Date:	Thu, 21 May 2015 19:02:28 -0300
From:	Ezequiel Garcia <ezequiel.garcia@...tec.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	<linux-kernel@...r.kernel.org>, <linux-mips@...ux-mips.org>,
	"Daniel Lezcano" <daniel.lezcano@...aro.org>,
	<devicetree@...r.kernel.org>,
	"Andrew Bresticker" <abrestic@...omium.org>,
	James Hartley <james.hartley@...tec.com>,
	James Hogan <james.hogan@...tec.com>,
	<Damien.Horsley@...tec.com>, <Govindraj.Raja@...tec.com>
Subject: Re: [PATCH 6/7] clocksource: Add Pistachio clocksource-only driver



On 05/21/2015 07:00 PM, Thomas Gleixner wrote:
> On Thu, 21 May 2015, Ezequiel Garcia wrote:
>> +static cycle_t clocksource_read_cycles(struct clocksource *cs)
>> +{
>> +	u32 counter, overflw;
>> +	unsigned long flags;
>> +
>> +	raw_spin_lock_irqsave(&lock, flags);
> 
> Hmm. Is that lock really necessary to read that counter? The
> clocksource is global. And if its actually used for timekeeping, the
> lock can get heavy contended.
> 

Yup, it is really (and sadly) necessary. The kernel hangs up completely
without it when the counter is accesed by more than one CPU.

Apparently, those two timer registers overflow and counter must be read
atomically.

>> +	overflw = gpt_readl(TIMER_CURRENT_OVERFLOW_VALUE, 0);
>> +	counter = gpt_readl(TIMER_CURRENT_VALUE, 0);
>> +	raw_spin_unlock_irqrestore(&lock, flags);
>> +
>> +	return ~(cycle_t)counter;
>> +}
> 
> Thanks,
> 
> 	tglx
> 

-- 
Ezequiel
--
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/