linux-kernel - RE: [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks

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Date:	Wed, 27 Apr 2011 14:04:25 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric B Munson" <emunson@...bm.net>
Cc:	<benh@...nel.crashing.org>, <a.p.zijlstra@...llo.nl>,
	<paulus@...ba.org>, <mingo@...e.hu>, <acme@...stprotocols.net>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
	<anton@...ba.org>, <stable@...nel.org>
Subject: RE: [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks

 
> On Wed, 27 Apr 2011, David Laight wrote:
> 
> > > But it isn't, and it doesn't have trouble with 2^32 - 1.  
> > 
> > what about:
> > prev = 0x00000001
> > val  = 0xffffffff
> 
> Result is 0xfffffffe and we are fine.

'delta' will be 0xfffffffe, but you need the function to
return zero - since the underlying counter has decremented
by 2. But 'prev > val' is false so the counter will
be increased by 2^32-2.

	David


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