linux-kernel - Re: [PATCH v4 02/17] sched_clock: Use seqcount instead of rolling our own

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Date:	Fri, 19 Jul 2013 10:03:28 +0100
From:	Will Deacon <will.deacon@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Christopher Covington <cov@...eaurora.org>
Subject: Re: [PATCH v4 02/17] sched_clock: Use seqcount instead of rolling
 our own

On Fri, Jul 19, 2013 at 12:21:15AM +0100, Stephen Boyd wrote:
> We're going to increase the cyc value to 64 bits in the near
> future. Doing that is going to break the custom seqcount
> implementation in the sched_clock code because 64 bit numbers
> aren't guaranteed to be atomic. Replace the cyc_copy with a
> seqcount to avoid this problem.
> 
> Cc: Russell King <linux@....linux.org.uk>
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> ---
>  kernel/time/sched_clock.c | 27 ++++++++-------------------
>  1 file changed, 8 insertions(+), 19 deletions(-)

Looks good to me. The current scheme would be very fiddly to extend to
64-bit values on 32-bit architectures without cheap atomic doubleword
accesses.

Acked-by: Will Deacon <will.deacon@....com>

Will
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