linux-kernel - Re: How is jiffies upation while holding spinlock in Uniprocessor or SMP environment?

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Message-ID: <CAG1O52UE53vA8=aXUuBBeSTpiu0R+XYarW09WUDScc5_aW1u0w@mail.gmail.com>
Date:	Fri, 30 Oct 2015 16:29:25 +0530
From:	raghu MG <raghumag@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Andrew Lunn <andrew@...n.ch>
Subject: Re: How is jiffies upation while holding spinlock in Uniprocessor or
 SMP environment?

Thank you for clarifying.

On Mon, Oct 26, 2015 at 4:18 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Mon, Oct 26, 2015 at 12:19:31PM +0530, raghu MG wrote:
>> Hello,
>> I have couple of questions.
>> 1) In case of UP how is jiffies(timer) get updated while holding spin
>> lock using spinlock_irq_save?
>
> It isn't, and...
>>
>> 2)My understanding is in smp environment jiffies updation cannot be
>> done on processor holding spinlock irrespective of spinlock API.So
>> timer interrupt should be enabled across all cores in SoC. Imagine a
>> scenario in quad core processor where 4 cores are holding 4 different
>> spinlocks & how is the timer interrupt for jiffies handled?
>
> You can't rely on jiffies being updated while holding a spinlock.
> Expecting jiffies to update while holding a spinlock is a bug.
>
> --
> FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
> according to speedtest.net.
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