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

[Metzger, Markus T]

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@xxxxxxx]
>Sent: Thursday, April 02, 2009 8:45 PM
>To: Metzger, Markus T
>Cc: tglx@xxxxxxxxxxxxx; hpa@xxxxxxxxx; markus.t.metzger@xxxxxxxxx; roland@xxxxxxxxxx;
>eranian@xxxxxxxxxxxxxx; oleg@xxxxxxxxxx; Villacis, Juan; ak@xxxxxxxxxxxxxxxxxx; linux-
>kernel@xxxxxxxxxxxxxxx
>Subject: Re: [patch 01/18] x86, bts: fix race when bts tracer is removed
>
>
>* markus.t.metzger@xxxxxxxxx <markus.t.metzger@xxxxxxxxx> 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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