linux-kernel - Re: [PATCH 5/5] perf tools: Fix data_read

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Date:	Tue, 19 May 2015 22:58:43 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCH 5/5] perf tools: Fix data_read_offset() file opening

On 19/05/2015 5:48 p.m., Namhyung Kim wrote:
> Hi Adrian,
>
> On Tue, May 19, 2015 at 04:05:46PM +0300, Adrian Hunter wrote:
>> Patch "perf tools: Protect dso cache fd with a mutex"
>> changed data_file_size() to open the data file if it
>> was not open.
>>
>> data_read_offset() was calling data_file_size() to read
>> the data file size, but data_file_size() can fail to
>> open the file because the binary_type has not been set up.
>>
>> The correct function to call is dso__data_size() which
>> uses dso__data_fd() to open the file correctly.
>
> Right, but I worried about the locking overhead.  By using
> dso__data_size() it'll call dso__data_fd() everytime which grabs the
> dso__data_open_lock that is a global mutex.  It can be a performance
> bottleneck on multi-thread report IMHO.
>
> I assumed that correct code should check the data fd or size before
> calling read function as I read the comment in dso.h file.
>
> What about adding the missing check in a proper place instead?

It looks to me like you need to change data_file_size() and
dso_cache__read() so that they open the fd properly i.e. call
dso__data_fd() instead of open_dso().

dso__data_fd() should not be taking a lock because the lock must
be held while the fd in being used.

Perhaps there should be dso__get_fd() and dso__put_fd() that lock/open
and the unlock.

But I will have to leave this to you because I am snowed with
my own problems.

>
> Thanks,
> Namhyung
>
>
>>
>> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
>> ---
>>   tools/perf/util/dso.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
>> index 1b96c8d..e248f56 100644
>> --- a/tools/perf/util/dso.c
>> +++ b/tools/perf/util/dso.c
>> @@ -756,7 +756,7 @@ off_t dso__data_size(struct dso *dso, struct machine *machine)
>>   static ssize_t data_read_offset(struct dso *dso, struct machine *machine,
>>   				u64 offset, u8 *data, ssize_t size)
>>   {
>> -	if (data_file_size(dso, machine))
>> +	if (dso__data_size(dso, machine) < 0)
>>   		return -1;
>>
>>   	/* Check the offset sanity. */
>> --
>> 1.9.1
>>
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