Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760408AbZDAQWY (ORCPT ); Wed, 1 Apr 2009 12:22:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1765148AbZDAQWL (ORCPT ); Wed, 1 Apr 2009 12:22:11 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:56525 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1764024AbZDAQWK (ORCPT ); Wed, 1 Apr 2009 12:22:10 -0400 Date: Wed, 1 Apr 2009 12:22:08 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: "K.Prasad" cc: Ingo Molnar , Linux Kernel Mailing List , Andrew Morton , Benjamin Herrenschmidt , Frederic Weisbecker , , Roland McGrath , Steven Rostedt Subject: Re: [Patch 00/11] Hardware Breakpoint interfaces In-Reply-To: <20090328084647.GB5297@in.ibm.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2075 Lines: 51 On Sat, 28 Mar 2009, K.Prasad wrote: > On Wed, Mar 25, 2009 at 03:48:31PM -0400, Alan Stern wrote: > > > + > > > +/* > > > + * Handle debug exception notifications. > > > + */ > > > +int __kprobes hw_breakpoint_handler(struct die_args *args) > > > +{ > > > + int i, rc = NOTIFY_DONE; > > > + struct hw_breakpoint *bp; > > > + /* The DR6 value is stored in args->err */ > > > + unsigned long dr7, dr6 = args->err; > > > > Please change this. (I should have changed it long ago, but I never > > got around to it.) Instead of passing the DR6 value in args->err, > > pass a pointer to the dr6 variable in do_debug(). That way the > > handler routines can turn off bits in that variable and do_debug() can > > see which bits remain set at the end. > > > > Of course, this will require a corresponding change to the > > post_kprobe_handler() routine. > > > > As I looked at the code with an intention of changing it, I don't find a > place - in hw_breakpoint_handler() and in functions called by > kprobe_exceptions_notify() where bits in dr6 are written into. That's because such writes wouldn't do any good in the old code! :-) > The thread-specific thread->debugreg6 is updated with causative bits in > ptrace_triggered() to help send signals to the user-space. I don't see a > user for the change you propose. For each breakpoint where we decide it's a case of lazy DR switching or we invoke a "triggered" callback, the corresponding bit in dr6 should be cleared. This is a way of indicating to do_debug() that the handler has taken care of these causes of the exception. Similarly, the kprobe routine should clear the single-step bit in dr6 when it handles a single-step exception. When the notifier chain completes, the only bits remaining in dr6 should be for events that still need to be handled. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/