linux-kernel - Re: [PATCH] tools/perf: fix the word selected in find_*

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Message-ID: <20160615124409.6985.qmail@ns.sciencehorizons.net>
Date:	15 Jun 2016 08:44:09 -0400
From:	"George Spelvin" <linux@...encehorizons.net>
To:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	maddy@...ux.vnet.ibm.com
Cc:	acme@...nel.org, adrian.hunter@...el.com, bp@...e.de,
	dsahern@...il.com, jolsa@...hat.com, linux@...izon.com,
	linux@...musvillemoes.dk, mpe@...erman.id.au, namhyung@...nel.org,
	wangnan0@...wei.com, yury.norov@...il.com
Subject: Re: [PATCH] tools/perf: fix the word selected in find_*_bit

Madhavan Srinivasan wrote:
> +#if (__BYTE_ORDER == __BIG_ENDIAN) && (BITS_PER_LONG != 64)
> +	tmp = addr[(((nbits - 1)/BITS_PER_LONG) - (start / BITS_PER_LONG))]
> +								^ invert;
> +#else
>  	tmp = addr[start / BITS_PER_LONG] ^ invert;
> +#endif

Than you for diagnosing this problem, but I don't think the fix
is correct.

1) It's not clear that all users of _find_next_bit and for_each_set_bit()
   want this change.
2) Is your code even correct?  I'd think you'd want addr[x ^ 1].  Are you
   sure you shpuld be reversing the whole array, and not just the halves of
   each 64-bit word?
3) You've now broken the case of 32-bit big-endian kernel.

I think the proper solution is uglier than this. :-(