linux-kernel - Re: perf: PERF_FLAG_FD_OUTPUT has been broken since 2.6.35

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Date:	Tue, 13 Jan 2015 10:36:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vince Weaver <vincent.weaver@...ne.edu>
Cc:	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: perf: PERF_FLAG_FD_OUTPUT has been broken since 2.6.35

On Fri, Jan 09, 2015 at 11:26:54AM -0500, Vince Weaver wrote:
> 
> On Fri, 9 Jan 2015, Peter Zijlstra wrote:
> > > So is this worth fixing seeing as apparently no one uses this feature?
> > 
> > I think there's a fair argument for removing it, Ingo, Acme?
> 
> could the functionality be replaced with a subsequent call to
> 	ioctl(PERF_EVENT_IOC_SET_OUTPUT)

Yes.

> Although I suppose there's a possibility for losing a small amount of data 
> or some other reason that PERF_FLAG_FD_OUTPUT was introduced in the first 
> place.

That's a natural race without solution. That is, because there is no
serialization between the sys_perf_event_open() and any possible acts of
data generation, we can't even talk about loosing data.

Even if it were all in a single syscall, there is no saying how long
that syscall would take to complete -- imagine its stuck on memory
allocation or whatnot. Similarly you could have issued the syscall a wee
bit later or whatnot.

> In addition, if we remove PERF_FLAG_FD_OUTPUT would there then be any 
> reason to keep PERF_FLAG_FD_NO_GROUP around?

Good point, I think there was another potential user of that proposed
recently, but I can't quite remember where or what. Let me try and go
find that.
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