linux-kernel - Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched

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, 03 Jun 2013 14:11:59 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, arm@...nel.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched_clock

On 06/03/13 02:39, Russell King - ARM Linux wrote:
> On Sat, Jun 01, 2013 at 11:39:41PM -0700, Stephen Boyd wrote:
>> +static unsigned long long notrace sched_clock_64(void)
>> +{
>> +	u64 cyc = read_sched_clock_64() - cd.epoch_ns;
>> +	return cyc * cd.mult;
> So, the use of cd.mult implies that the return value from
> read_sched_clock_64() is not nanoseconds but something else.  But then
> we subtract it from the nanoseconds epoch - which has to be nanoseconds
> because you simply return that when suspended.

You're right, it is confusing and broken. I was thinking we may need a
union for epoch_ns but I will try to make it always nanoseconds and see
if that makes the code clearer.

>
>> +}
>> +
>> +void __init
>> +setup_sched_clock_64(u64 (*read)(void), int bits, unsigned long rate)
>> +{
>> +	if (cd.rate > rate)
>> +		return;
>> +
>> +	BUG_ON(bits <= 32);
>> +	WARN_ON(!irqs_disabled());
>> +	read_sched_clock_64 = read;
>> +	sched_clock_func = sched_clock_64;
>> +	cd.rate = rate;
>> +	cd.mult = NSEC_PER_SEC / rate;
> Here, you don't check that the (2^bits) * mult results in a wrap of the
> resulting 64-bit number, which is a _basic_ requirement for sched_clock
> (hence all the code for <=32bit clocks, otherwise we wouldn't need this
> complexity in the first place.)

Ok I will use clocks_calc_mult_shift() here.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation

--
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/