linux-kernel - Re: [PATCH] perf tools: enable close-on-exec flag on perf file descriptor

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Message-ID: <1389042095.13828.30.camel@localhost.localdomain>
Date:	Mon, 06 Jan 2014 22:01:35 +0100
From:	Yann Droneaud <ydroneaud@...eya.com>
To:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Michael Ellerman <michael@...erman.id.au>,
	linux-kernel@...r.kernel.org, Yann Droneaud <ydroneaud@...eya.com>
Subject: Re: [PATCH] perf tools: enable close-on-exec flag on perf file
 descriptor

Hi,

Le lundi 06 janvier 2014 à 11:43 -0300, Arnaldo Carvalho de Melo a
écrit :
> Em Mon, Jan 06, 2014 at 12:24:36PM +0100, Peter Zijlstra escreveu:

> > acme, ACK? I was thinking I'd keep these two patches together so the
> > entire things lands in tip in one go?
> 
> Nope, it should notice the EINVAL, drop the flag that doesn't work on
> older kernels, retry, so that new tools continue to work on older
> kernels, with yet another fallback.
> 

But it may be difficult to distinguish the 'origin' of EINVAL: is it
from FD_CLOEXEC flag or from an unsupported parameter in the attributes.

> Please take a look at __perf_evsel__open(), probably it will be best to
> add a flag to perf_missing_features, like the one we have for the
> perf_event_attr.mmap2 flag, so that we fail just once, etc.
> 

Unfortunately perf_event_open() is called in multiple places, not only
in __perf_evsel__open(). So a more generic solution should be designed.

Is something like the function proposed in message
<1389022310.13828.9.camel@...alhost.localdomain> [1]
ok to be added to its own module:

static int cloexec = PERF_FLAG_FD_CLOEXEC;

int perf_flag_fd_cloexec(void)
{
    static int probed;

    if (!probed) {
        struct perf_event_attr attr = { 0 };
        int fd = perf_event_open(&attr, 0, -1, -1,
PERF_FLAG_FD_CLOEXEC);
        probed = 1;
        if (fd >= 0)
            close(fd);
        else
            cloexec = 0;
     }

     return cloexec;
}

This function should be used to build the flag passed to
perf_event_open().

Regards.

[1] 
http://lkml.kernel.org/r/1389022310.13828.9.camel@localhost.localdomain

-- 
Yann Droneaud
OPTEYA


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