linux-kernel - Re: [Patch 02/12] Introducing generic hardware breakpoint handler interfaces

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Date:	Tue, 26 May 2009 08:52:52 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Steven Rostedt <rostedt@...dmis.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 02/12] Introducing generic hardware breakpoint handler
	interfaces

On Mon, May 25, 2009 at 08:36:36PM +0200, Frederic Weisbecker wrote:
> On Sat, May 16, 2009 at 05:55:52AM +0530, K.Prasad wrote:
> > This patch introduces the generic Hardware Breakpoint interfaces for both user
> > and kernel space requests.
> > 
> > Original-patch-by: Alan Stern <stern@...land.harvard.edu>
> > Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com>
> > Reviewed-by: Alan Stern <stern@...land.harvard.edu>
> > ---
> >  arch/Kconfig           |    4 
> >  kernel/Makefile        |    1 
> >  kernel/hw_breakpoint.c |  369 +++++++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 374 insertions(+)
> > 
> > +/**
> > + * register_kernel_hw_breakpoint - register a hardware breakpoint for kernel space
> > + * @bp: the breakpoint structure to register
> > + *
> > + * @bp.info->name or @bp.info->address, @bp.info->len, @bp.info->type and
> > + * @bp->triggered must be set properly before invocation
> > + *
> > + */
> > +int register_kernel_hw_breakpoint(struct hw_breakpoint *bp)
> > +{
> > +	int rc;
> > +
> > +	rc = arch_validate_hwbkpt_settings(bp, NULL);
> > +	if (rc)
> > +		return rc;
> > +
> > +	spin_lock_bh(&hw_breakpoint_lock);
> > +
> > +	rc = -EINVAL;
> > +	/* Check if we are over-committing */
> > +	if ((hbp_kernel_pos > 0) && (!hbp_user_refcount[hbp_kernel_pos-1])) {
> 
> 
> 
> If hbp_kernel_pos == 0, shouldn't it return -ENOSPC ?
> If several kernel users try to register a breakpoint, and there is no
> more room for one of them, then we know it has failed not because
> of an error in the breakpoint instance but because of a lack of
> resources.
> 
> Frederic.
>

Agreed. It's done correctly in the case of register_user_hw_breakpoint()
but not here. Here's a tiny patch that corrects it.

 
Modify the return code in register_kernel_hw_breakpoint()

Modify the return code in register_kernel_hw_breakpoint() to return -ENOSPC
if we are error-returning due to lack of free HW breakpoint registers.

Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com>
---
 kernel/hw_breakpoint.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6-tip.hbkpt/kernel/hw_breakpoint.c
===================================================================
--- linux-2.6-tip.hbkpt.orig/kernel/hw_breakpoint.c
+++ linux-2.6-tip.hbkpt/kernel/hw_breakpoint.c
@@ -312,7 +312,7 @@ int register_kernel_hw_breakpoint(struct
 
 	spin_lock_bh(&hw_breakpoint_lock);
 
-	rc = -EINVAL;
+	rc = -ENOSPC;
 	/* Check if we are over-committing */
 	if ((hbp_kernel_pos > 0) && (!hbp_user_refcount[hbp_kernel_pos-1])) {
 		hbp_kernel_pos--;
--
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