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

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Date:	Fri, 29 May 2009 09:54:51 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	David Gibson <dwg@....ibm.com>,
	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

On Fri, 29 May 2009, 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.

That's exactly why I wrote it.

Now, if we could guarantee that io_bitmap_ptr was always NULL upon
entry to copy_thread(), the statement could of course be removed.  In
fact it probably is NULL, since otherwise the current code wouldn't
work properly.

Alan Stern

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