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

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Date:	Tue, 19 May 2015 23:48:18 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
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

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?

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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