linux-kernel - RE: debugctl msr

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Date:	Fri, 21 Nov 2008 08:22:30 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	"eranian@...il.com" <eranian@...il.com>,
	Markus Metzger <markus.t.metzger@...glemail.com>
CC:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: debugctl msr

>-----Original Message-----
>From: stephane eranian [mailto:eranian@...glemail.com] 
>Sent: Donnerstag, 20. November 2008 22:19
>To: Markus Metzger

>- in ds_put_context(), you need to mark task->thread.ds_ctx = NULL or
>this_system_context = NULL when you are
>  done, otherwise a subsequent session on the same task or CPU will
>think there is already a context allocated but
>  the pointer will be stale.

This should happen in:
    *(context->this) = NULL;

Did you see a context without owners?


>- in ptrace.c:ptrace_disable(), you systematically invoke ds_release()
>without checking if TIF_BTS_TRACE_TS
>  is set. That causes extraneous calls to ds_release() which messes up
>the reference counting if PEBS is in use.

That TIF only triggers the recording of scheduling timestamps.

That ds_release_bts() call should:
- get the context
- fail the ownership validation in ds_validate()
- put the context

If ds_validate() is disabled, it would put the context twice.


I am currently working on a patch to replace the validation
mechanism by using a handle returned from ds_request().

I also added the interrupt threshold to the parameters of ds_request().
It will be fix for one tracing session, if that's OK with you.


thanks and regards,
markus.
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