linux-kernel - Re: How do I increment a per-CPU variable without warning?

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Message-ID: <20140417182106.GP4496@linux.vnet.ibm.com>
Date:	Thu, 17 Apr 2014 11:21:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, tj@...nel.org,
	grygorii.strashko@...com, peterz@...radead.org
Subject: Re: How do I increment a per-CPU variable without warning?

On Thu, Apr 17, 2014 at 12:53:28PM -0500, Christoph Lameter wrote:
> On Thu, 17 Apr 2014, Paul E. McKenney wrote:
> 
> > Fair enough!  I resent the patch with your Ack to Tejun.
> 
> Also note that you may want to use
> 
> 	this_cpu_inc
> 
> instead of raw_cpu_inc.
> 
> this_cpu_inc will not disable preemption or anything on x86 but just
> create a single instruction using instruction atomicity to avoid the
> preempt on/off sequence.
> 
> 
> On platforms that cannot emit such an instruction it will fallback to
> disable interrupts for the sequence of instructions that increments the
> value.
> 
> With such an approach incrementing the counter should be much safer. If
> the other arch want to avoid irq on/off sequences then they can override
> the fallback to use atomics or whatever the processor architecture permits
> to avoid the overhead of interrupt on / off.

Fair enough, but in this case I don't need it to be safe.

							Thanx, Paul

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