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

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Message-ID: <20151026104854.GC8644@n2100.arm.linux.org.uk>
Date:	Mon, 26 Oct 2015 10:48:54 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	raghu MG <raghumag@...il.com>
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?

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.

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