linux-kernel - Re: [RFC PATCH] percpu: Prevent endless loop if there is no unallocated region

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Message-ID: <20150810163638.GC23408@mtj.duckdns.org>
Date:	Mon, 10 Aug 2015 12:36:38 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Christoph Lameter <cl@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] percpu: Prevent endless loop if there is no
 unallocated region

Hello,

On Sun, Aug 09, 2015 at 05:17:39AM -0700, Guenter Roeck wrote:
> Qemu tests with unicore32 show memory management code entering an endless
> loop in pcpu_alloc(). Bisect points to commit a93ace487a33 ("percpu: move
> region iterations out of pcpu_[de]populate_chunk()"). Code analysis
> identifies the following relevant changes.
> 
> -       rs = page_start;
> -       pcpu_next_pop(chunk, &rs, &re, page_end);
> -
> -       if (rs != page_start || re != page_end) {
> +       pcpu_for_each_unpop_region(chunk, rs, re, page_start, page_end) {
> 
> For unicore32, values were page_start==0, page_end==1, rs==0, re==1.
> This worked fine with the old code. With the new code, however, the loop
> is always entered. Debugging information added into the loop shows
> an endless repetition of
> 
> in loop chunk c5c53100 populated 0xff rs 1 re 2 page start 0 page end 1
> in loop chunk c5c53100 populated 0xff rs 1 re 2 page start 0 page end 1
> in loop chunk c5c53100 populated 0xff rs 1 re 2 page start 0 page end 1
> in loop chunk c5c53100 populated 0xff rs 1 re 2 page start 0 page end 1

That's a bug in the find bit functions in unicore32.  If @offset >=
@end, it should return @end, not @offset.

Thanks.

-- 
tejun
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