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

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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E9271804B@irsmsx504.ger.corp.intel.com>
Date:	Fri, 3 Apr 2009 07:19:56 +0100
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
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

>-----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)."


>> +
>> +	if (tracer && (tracer->flags & BTS_TIMESTAMPS)) {
>> +		struct bts_struct ts = {
>> +			.qualifier = qualifier,
>> +			.variant.timestamp.jiffies = jiffies_64,
>> +			.variant.timestamp.pid = task->pid
>> +		};
>> +		bts_write(tracer, &ts);
>> +	}
>
>Why do we have .variant.timestamp.pid ? A PID is not a timestamp. It
>might be .event.jiffies and .event.pid perhaps.

OK.


>Also, the whole function could be cleaned up by:
>
> 1) returning early if !tracer || !(tracer->flags & BTS_TIMESTAMPS).
>
> 2) Doing a cleaner initialization - something like:
>
>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?

thanks,
markus.
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