Date: Tue, 15 Dec 2009 22:00:55 +1100
From: Paul Mackerras <paulus@samba.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       Michael Neuling <mikey@neuling.org>
Subject: Re: [PATCH 1/2] perf_event: Fix incorrect range check on cpu number
Message-ID: <20091215110055.GC18661@brick.ozlabs.ibm.com>
References: <20091215084032.GA18661@brick.ozlabs.ibm.com>
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On Tue, Dec 15, 2009 at 11:31:32AM +0100, Peter Zijlstra wrote:

> On Tue, 2009-12-15 at 19:40 +1100, Paul Mackerras wrote:
> > It is quite legitimate for CPUs to be numbered sparsely, meaning that
> > it possible for an online CPU to have a number which is greater than
> > the total count of possible CPUs.
> > 
> > Currently find_get_context() has a sanity check on the cpu number
> > where it checks it against num_possible_cpus().  This test can fail
> > for a legitimate cpu number if the cpu_possible_mask is sparsely
> > populated.
> > 
> > This fixes the problem by checking the CPU number against
> > nr_cpumask_bits instead, since that is the appropriate check to ensure
> > that the cpu number is same to pass to cpu_isset() subsequently.
> 
> Cute, do you actually have hardware that does this?

Yeah, Mikey ran across this on a POWER7 box here.

Paul.
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