linux-kernel - Re: [PATCH 40/42] perf callchain: Save eh/debug frame offset for dwarf unwind

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Date:	Thu, 29 Jan 2015 16:22:05 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jiri Olsa <jolsa@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Stephane Eranian <eranian@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 40/42] perf callchain: Save eh/debug frame offset for
 dwarf unwind

Em Thu, Jan 29, 2015 at 05:07:21PM +0900, Namhyung Kim escreveu:
> When libunwind tries to resolve callchains it needs to know the offset
> of .eh_frame_hdr or .debug_frame to access the dso.  Since it calls
> dso__data_fd(), it'll try to grab dso->lock everytime for same
> information.  So save it to dso_data struct and reuse it.
> 
> Note that there's a window between dso__data_fd() and actual use of
> the fd.  The fd could be closed by other threads to deal with the open
> file limit in dso cache code.  But I think it's ok since in that case
> elf_section_offset() will return 0 so it'll be tried in next acess.

Thanks, applied after rewriting the changelog to read as:

---
    perf callchain: Cache eh/debug frame offset for dwarf unwind
    
    When libunwind tries to resolve callchains it needs to know the
    offset of .eh_frame_hdr or .debug_frame to access the dso.
    
    Since it will always return the same result for a given DSO, just
    cache the result as an optimization.
---

- Arnaldo
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