Messages in this thread | | | From | "Metzger, Markus T" <> | Date | Fri, 3 Apr 2009 07:19:56 +0100 | Subject | RE: [patch 01/18] x86, bts: fix race when bts tracer is removed |
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>-----Original Message----- >From: Ingo Molnar [mailto:mingo@elte.hu] >Sent: Thursday, April 02, 2009 8:45 PM >To: Metzger, Markus T >Cc: tglx@linutronix.de; hpa@zytor.com; markus.t.metzger@gmail.com; roland@redhat.com; >eranian@googlemail.com; oleg@redhat.com; Villacis, Juan; ak@linux.jf.intel.com; linux- >kernel@vger.kernel.org >Subject: Re: [patch 01/18] x86, bts: fix race when bts tracer is removed > > >* markus.t.metzger@intel.com <markus.t.metzger@intel.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?
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