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

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Message-ID: <20140416160621.GX4496@linux.vnet.ibm.com>
Date:	Wed, 16 Apr 2014 09:06:21 -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 Wed, Apr 16, 2014 at 10:08:03AM -0500, Christoph Lameter wrote:
> On Tue, 15 Apr 2014, Paul E. McKenney wrote:
> 
> > Hello, Christoph,
> >
> > I have a patch that currently uses __this_cpu_inc_return() to increment a
> > per-CPU variable, but without preemption disabled.  Of course, given that
> > preemption is enabled, it might well end up picking up one CPU's counter,
> > adding one to it, then storing the result into some other CPU's counter.
> > But this is OK, the test can be probabilistic.  And when I run this
> > against v3.14 and earlier, it works fine.
> 
> We introduced raw_cpu_inc_return to squish these warnings.

Cool, this is a good short-term fix.

> > This is arguably better than the original __this_cpu_read() because it
> > avoids overflow, but I thought I should check to see if there was some
> > better way to do this.
> 
> If this is supposed to be totally race safe then you must disable
> preemption.

Understood!

							Thanx, Paul

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