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

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Message-ID: <20090402184514.GC843@elte.hu>
Date:	Thu, 2 Apr 2009 20:45:14 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	markus.t.metzger@...el.com
Cc:	tglx@...utronix.de, hpa@...or.com, markus.t.metzger@...il.com,
	roland@...hat.com, eranian@...glemail.com, oleg@...hat.com,
	juan.villacis@...el.com, 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()?

> +
> +	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.

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?

	Ingo
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