linux-kernel - Re: [patch 01/18] x86, bts: fix race when bts tracer is removed

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Date:	Fri, 3 Apr 2009 13:29:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Metzger, Markus T" <markus.t.metzger@...el.com>
Cc:	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>,
	"roland@...hat.com" <roland@...hat.com>,
	"eranian@...glemail.com" <eranian@...glemail.com>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"Villacis, Juan" <juan.villacis@...el.com>,
	"ak@...ux.jf.intel.com" <ak@...ux.jf.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [patch 01/18] x86, bts: fix race when bts tracer is removed


* Metzger, Markus T <markus.t.metzger@...el.com> wrote:

> >-----Original Message-----
> >From: Ingo Molnar [mailto:mingo@...e.hu]
> >Sent: Thursday, April 02, 2009 8:45 PM
> >To: Metzger, Markus T
> >Cc: tglx@...utronix.de; hpa@...or.com; markus.t.metzger@...il.com; roland@...hat.com;
> >eranian@...glemail.com; oleg@...hat.com; Villacis, Juan; ak@...ux.jf.intel.com; linux-
> >kernel@...r.kernel.org
> >Subject: Re: [patch 01/18] x86, bts: fix race when bts tracer is removed
> >
> >
> >* markus.t.metzger@...el.com <markus.t.metzger@...el.com> wrote:
> >
> >> +static inline void ds_take_timestamp(struct ds_context *context,
> >> +				     enum bts_qualifier qualifier,
> >> +				     struct task_struct *task)
> >> +{
> >> +	struct bts_tracer *tracer = context->bts_master;
> >> +	barrier();
> >
> >why the barrier()?
> 
> See http://lkml.org/lkml/2009/3/31/544
> 
> Oleg: "In theory, we need barrier() after reading ->bts_master.
> 
> (actually, I did see the bug reports when the compiler read the pointer
>  twice with the code like above)."

Please convert this piece of non-trivial information into a small 
two-sentence blurb and put it into a comment block.

> >struct bts_struct ts = {
> >	.qualifier		= qualifier,
> >	.variant.event.jiffies	= jiffies_64,
> >	.variant.event.pid	= task->pid
> >};
> >
> >Also, raw use of jiffies_64 is buggy and racy. Why does this use
> >jiffies to begin with - why not some finer grained time?
> 
> What would be a good time to use?

ktime_get() would be the primary candidate. (Or, perhaps, if 
performance is really an issue then trace_clock() or 
trace_clock_global().)

	Ingo
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