linux-kernel - Re: [Patch 06/12] Use the new wrapper routines to access debug registers in process/thread code

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Message-ID: <c62985530905290707y7a44a824p30f0857b31129b4@mail.gmail.com>
Date:	Fri, 29 May 2009 16:07:07 +0200
From:	Frédéric Weisbecker <fweisbec@...il.com>
To:	prasad@...ux.vnet.ibm.com
Cc:	David Gibson <dwg@....ibm.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@....ibm.com>,
	maneesh@...ux.vnet.ibm.com, Roland McGrath <roland@...hat.com>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: Re: [Patch 06/12] Use the new wrapper routines to access debug 
	registers in process/thread code

2009/5/29 K.Prasad <prasad@...ux.vnet.ibm.com>:
> On Fri, May 29, 2009 at 12:49:03PM +0200, Frederic Weisbecker wrote:
>> On Fri, May 29, 2009 at 02:31:46PM +0530, K.Prasad wrote:
>> > On Thu, May 28, 2009 at 04:42:38PM +1000, David Gibson wrote:
>> > > On Mon, May 11, 2009 at 05:23:44PM +0530, K.Prasad wrote:
>> > > > From: Alan Stern <stern@...land.harvard.edu>
>> > > >
>> > > > This patch enables the use of abstract debug registers in
>> > > > process-handling routines.
>> > >
>> > > [snip]
>> > > >
>> > > > +       p->thread.io_bitmap_ptr = NULL;
>> > >
>> > > Why is manipulating the io_bitmap_ptr relevant to debug register
>> > > handling?
>> >
>> > I *re-read* the patch but was unable to find how this change had sneaked
>> > in. It shouldn't be there although it is harmless.
>>
>>
>> When I reviewed this patch, I also ended stucked on it.
>> But actually I guess I found the sense, this is only for
>> convenience.
>>
>> Look at the current copy_thread() in arch/x86/kernel/process32.c
>>
>> If p->thread.io_bitmap_ptr fails to be duplicated, we set
>> p->thread.io_bitmap_max = 0 and return -ENOMEM
>>
>> Now look at the patch.
>> If we fail to copy the hardware thread virtual registers we
>> want to exit with io_bitmap_ptr = NULL
>> If we fail to copy the io_bitmap, we want to free the breakpoint
>> and exit.
>> If we fail further, we want to free breakpoints and io_bitmap_ptr
>>
>> The out section then tries to:
>>
>> -free the breakpoints
>> -free p->thread.io_bitmap_ptr
>>
>>
>> So it's important to set io_bitmap_ptr to NULL so that
>> we know whether we have to release it or not.
>>
>>
>
> aah...yes. It tricked me! It is needed to bring the desired
> error-return behaviour of copy_thread(). Please ignore this patch (the
> updation of the comments can be brought in through a separate
> enhancement patch...see below).


Ok.


>> > Hi Frederic,
>> >     I am attaching a new version of this patch 06/12 that:
>> >
>> > - removes the line that assigns NULL to "p->thread.io_bitmap_ptr"
>>
>>
>> Dangerous. Unless p->thread.io_bitmap_ptr is already zeroed out
>> at this stage?
>>
>>
>> > - Updates the comment in __switch_to() function which was stale (was
>> >   relevant when 'last_debugged_task' was used to detect lazy debug
>> >   register switching).
>> >
>> > Kindly integrate this version in lieu of the older version sent here:
>> > http://lkml.org/lkml/2009/5/21/149.
>>
>>
>> Ok. Well it would be nice if you resend the whole series actually :)
>> Do you have another fix pending?
>>
>> Thanks!
>>
>
> In the process of responding to David Gibson's comments I agreed to
> a few minor/cosmetic changes - say like updation of comments, renaming
> functions or variables, etc.
>
> Given that the patchset is on the verge of integration into -tip tree, I
> would prefer them to be done through a separate patch (on -tip) for
> enhancement. Kindly let me know what you think about proceeding this
> way.


Well, indeed it would be easier for you to start iterating with a merged base.
But I would prefer to pull-request the patchset to Ingo once the pending fixes
are sent.

So to start the integration of this, I can apply the current patches in my tree,
based on tracing/core. And once you have the minor fixes addressing David's
comments, I also apply them and send the whole to Ingo.

Ok?
But still could you resend me the whole patchset you have, including
the fixes already posted, so that I don't mess up through several versions.

And I guess I could apply them very soon.

Thanks.


> Thanks,
> K.Prasad
>
>
>
>
>
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