Re: [PATCH 5/5] perf tools: Fix data_read_offset() file opening

[Arnaldo Carvalho de Melo]

Em Wed, May 20, 2015 at 09:44:37AM +0900, Namhyung Kim escreveu:
> 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.

Ok, I have Adrian's patch in my perf/core branch, tomorrow I'll check
what you guys ended up agreeing on so that I can do the adjustments
before pushing to Ingo, ok?
 
> > 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 ;-)

:-)

- Arnaldo
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