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

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Date:	Wed, 20 May 2015 09:44:37 +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

On Tue, May 19, 2015 at 10:58:43PM +0300, Adrian Hunter wrote:
> 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().

OK, I'll change it to have a proper binary type before calling
open_dso() like dso__data_fd() does.  But I still think it's good to
open data fd explicitly before reading from dso cache.


> 
> 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.

That is already a part of my patchset but not merged yet.


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

No problem.  I can see you're doing hardwork ;-)

Thanks,
Namhyung
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