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

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Date:	Wed, 15 Jun 2016 22:51:27 +0300
From:	Yury Norov <ynorov@...iumnetworks.com>
To:	Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
CC:	<linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Borislav Petkov <bp@...e.de>, David Ahern <dsahern@...il.com>,
	George Spelvin <linux@...izon.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Wang Nan <wangnan0@...wei.com>,
	Yury Norov <yury.norov@...il.com>,
	Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH] tools/perf: fix the word selected in find_*_bit

Hi Madhavan,

On Wed, Jun 15, 2016 at 05:12:53PM +0530, Madhavan Srinivasan wrote:
> When decoding the perf_regs mask in regs_dump__printf(),
> we loop through the mask using find_first_bit and find_next_bit functions.
> And mask is of type "u64". But "u64" is send as a "unsigned long *" to
> lib functions along with sizeof().
> 
> While the exisitng code works fine in most of the case, when using a 32bit perf
> on a 64bit kernel (Big Endian), we end reading the wrong word. In find_first_bit(),
> one word at a time (based on BITS_PER_LONG) is loaded and
> checked for any bit set. In 32bit BE userspace,
> BITS_PER_LONG turns out to be 32, and for a mask value of
> "0x00000000000000ff", find_first_bit will return 32, instead of 0.
> Reason for this is that, value in the word0 is all zeros and value
> in word1 is 0xff. Ideally, second word in the mask should be loaded
> and searched. Patch swaps the word to look incase of 32bit BE.

I think this is not a problem of find_bit() at all. You have wrong
typecast as the source of problem (tools/perf/util/session.c"):

940 static void regs_dump__printf(u64 mask, u64 *regs)
941 {
942         unsigned rid, i = 0;
943 
944         for_each_set_bit(rid, (unsigned long *) &mask, sizeof(mask) * 8) {
                                          ^^^^ Here ^^^^
945                 u64 val = regs[i++];
946 
947                 printf(".... %-5s 0x%" PRIx64 "\n",
948                        perf_reg_name(rid), val);
949         }
950 }

But for some reason you change correct find_bit()...

Though proper fix is like this for me:

static void regs_dump__printf(u64 mask, u64 *regs)
{
        unsigned rid, i = 0;
        unsigned long _mask[sizeof(mask)/sizeof(unsigned long)];

        _mask[0] = mask & ULONG_MAX;
        if (sizeof(mask) > sizeof(unsigned long))
                _mask[1] = mask >> BITS_PER_LONG;

        for_each_set_bit(rid, _mask, sizeof(mask) * BITS_PER_BYTE) {
                u64 val = regs[i++];

                printf(".... %-5s 0x%" PRIx64 "\n",
                       perf_reg_name(rid), val);
        }
}

Maybe there already is some macro doing the conversion for you...

Yury.

> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> Cc: Adrian Hunter <adrian.hunter@...el.com>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: David Ahern <dsahern@...il.com>
> Cc: George Spelvin <linux@...izon.com>
> Cc: Jiri Olsa <jolsa@...hat.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
> Cc: Wang Nan <wangnan0@...wei.com>
> Cc: Yury Norov <yury.norov@...il.com>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Signed-off-by: Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
> ---
>  tools/lib/find_bit.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/tools/lib/find_bit.c b/tools/lib/find_bit.c
> index 9122a9e80046..996b3e04324f 100644
> --- a/tools/lib/find_bit.c
> +++ b/tools/lib/find_bit.c
> @@ -37,7 +37,12 @@ static unsigned long _find_next_bit(const unsigned long *addr,
>  	if (!nbits || start >= nbits)
>  		return nbits;
> 
> +#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
> 
>  	/* Handle 1st word. */
>  	tmp &= BITMAP_FIRST_WORD_MASK(start);
> @@ -48,7 +53,12 @@ static unsigned long _find_next_bit(const unsigned long *addr,
>  		if (start >= nbits)
>  			return nbits;
> 
> +#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
>  	}
> 
>  	return min(start + __ffs(tmp), nbits);
> @@ -75,8 +85,15 @@ unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
>  	unsigned long idx;
> 
>  	for (idx = 0; idx * BITS_PER_LONG < size; idx++) {
> +#if (__BYTE_ORDER == __BIG_ENDIAN) && (BITS_PER_LONG != 64)
> +		if (addr[(((size-1)/BITS_PER_LONG) - idx)])
> +			return min(idx * BITS_PER_LONG +
> +				__ffs(addr[(((size-1)/BITS_PER_LONG) - idx)]),
> +									size);
> +#else
>  		if (addr[idx])
>  			return min(idx * BITS_PER_LONG + __ffs(addr[idx]), size);
> +#endif
>  	}
> 
>  	return size;
> --
> 1.9.1