linux-kernel - Re: perf record: why we used type casting of (uint64

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Message-ID: <CABPqkBQUDkW0qnf30Fv-dTgRCHr5D4tT43xuhHso88FC4M5Qiw@mail.gmail.com>
Date:	Fri, 25 May 2012 10:20:49 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: perf record: why we used type casting of (uint64_t *) instead of int

On Fri, May 25, 2012 at 7:27 AM, Anshuman Khandual
<khandual@...ux.vnet.ibm.com> wrote:
> This code is breaking in powerpc systems.
>
> (1) 'opt->value' gets updated inside the function parse_branch_stack via
>    dereferencing a (uint64_t *) type casted pointer.
>
> (2) But the value is not accessible when we again use opt->value via
>    dereferencing a (int *) type casted pointer.
>
> (3) As a result record.opts.branch_stack remains 0 and unchanged by parse_branch_stack
>
> This is caused by bit representation of 'uint64_t' and 'int' in powerpc systems. Bytes update
> for the data (when accessed trough (uint64_t *) casting) is no longer available to the
> data when accessed through (int *) type casting. Verified this from bit representation of
> the data (accessed through both type casting methods).
>
> However this problem does not seem to be present on an Intel box. Integer dereferencing of
> the opt->value still gives the value which was updated as (uint64_t).
>
> All this problem would not have been there if we had used (int *) instead of (uint64_t *) in
> the first place inside parse_branch_stack function.
>
The bug is that in struct record_opts, branch_stack is declared int
instead of u64.
I can post a patch to fix that. The value is eventually passed to struct
perf_event_attr.branch_sample_type  which is defined as u64.

I can post a patch to fix that.

Thanks for catching this.

> On Thursday 24 May 2012 02:51 PM, Anshuman Khandual wrote:
>
>> Hey Stephane,
>>
>> Just wondering why we used the type casting of (uint64_t *) on a data
>> which is defined as "int" in the structure of "perf_record_opts".
>>
>> struct perf_record_opts {
>>         struct perf_target target;
>>         bool         call_graph;
>>         bool         group;
>>         bool         inherit_stat;
>>         bool         no_delay;
>>         bool         no_inherit;
>>         bool         no_samples;
>>         bool         pipe_output;
>>         bool         raw_samples;
>>         bool         sample_address;
>>         bool         sample_time;
>>         bool         sample_id_all_missing;
>>         bool         exclude_guest_missing;
>>         bool         period;
>>         unsigned int freq;
>>         unsigned int mmap_pages;
>>         unsigned int user_freq;
>>         int          branch_stack;
>>         u64          default_interval;
>>         u64          user_interval;
>> };
>>
>> static int
>> parse_branch_stack(const struct option *opt, const char *str, int unset)
>> {
>> #define ONLY_PLM \
>>         (PERF_SAMPLE_BRANCH_USER        |\
>>          PERF_SAMPLE_BRANCH_KERNEL      |\
>>          PERF_SAMPLE_BRANCH_HV)
>>
>>         uint64_t *mode = (uint64_t *)opt->value;
>> --
>> Regards
>> Anshuman Khandual
>>
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>
>
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