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

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Message-Id: <37638e17-ab3f-2045-d3f6-faff11e8e87c@linux.vnet.ibm.com>
Date:	Thu, 16 Jun 2016 12:45:55 +0530
From:	Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
To:	Yury Norov <ynorov@...iumnetworks.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



On Thursday 16 June 2016 01:21 AM, Yury Norov wrote:
> 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...

Agreed, but reason for proposing fix in lib side is to avoid conversion
on each case (if any in future).

I will repost the fix as suggested.

Maddy
> 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