Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754157Ab0A0K2o (ORCPT ); Wed, 27 Jan 2010 05:28:44 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752756Ab0A0K2n (ORCPT ); Wed, 27 Jan 2010 05:28:43 -0500 Received: from e23smtp04.au.ibm.com ([202.81.31.146]:43781 "EHLO e23smtp04.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752674Ab0A0K2m (ORCPT ); Wed, 27 Jan 2010 05:28:42 -0500 Date: Wed, 27 Jan 2010 15:58:26 +0530 From: "K.Prasad" To: Frederic Weisbecker Cc: LKML , Ingo Molnar , Alan Stern Subject: Re: [RFC Patch 2/2][Bugfix][x86][hw-breakpoint] Fix return-code to notifier chain in hw_breakpoint_handler Message-ID: <20100127102826.GA3460@in.ibm.com> Reply-To: prasad@linux.vnet.ibm.com References: <20091226175533.149765731@linux.vnet.ibm.com> <20091226182833.GC9494@in.ibm.com> <20100125221101.GE5087@nowhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100125221101.GE5087@nowhere> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3412 Lines: 103 On Mon, Jan 25, 2010 at 11:11:04PM +0100, Frederic Weisbecker wrote: > On Sat, Dec 26, 2009 at 11:58:33PM +0530, K.Prasad wrote: > > The hw-breakpoint handler will return NOTIFY_DONE for user-space breakpoints > > to generate SIGTRAP signal (and not for kernel-space addresses). > > > Please tell a bit more in your changelogs. It took me some time > to guess whether this is a fix or not. > Sorry about that...will add a descriptive changelog. > And this is not a fix but an optimization because SIGTRAP > is only sent if needed. > > Here is what happens in do_debug() after handling the > breakpoint: > > if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS)) > send_sigtrap(tsk, regs, error_code, si_code); > > This can only happen if we took the ptrace handler path. > Agreed...signals are prevented as above...except that the notifier semantics aren't properly used (NOTIFY_DONE vs NOTIFY_STOP). > Also: > > > > Signed-off-by: K.Prasad > > --- > > arch/x86/kernel/hw_breakpoint.c | 9 +++++++-- > > 1 file changed, 7 insertions(+), 2 deletions(-) > > > > Index: linux-2.6-tip/arch/x86/kernel/hw_breakpoint.c > > =================================================================== > > --- linux-2.6-tip.orig/arch/x86/kernel/hw_breakpoint.c > > +++ linux-2.6-tip/arch/x86/kernel/hw_breakpoint.c > > @@ -502,8 +502,6 @@ static int __kprobes hw_breakpoint_handl > > rcu_read_lock(); > > > > bp = per_cpu(bp_per_reg[i], cpu); > > - if (bp) > > - rc = NOTIFY_DONE; > > /* > > * Reset the 'i'th TRAP bit in dr6 to denote completion of > > * exception handling > > @@ -517,6 +515,13 @@ static int __kprobes hw_breakpoint_handl > > rcu_read_unlock(); > > break; > > } > > + /* > > + * Further processing in do_debug() is needed for a) user-space > > + * breakpoints (to generate signals) and b) when the system has > > + * taken exception due to multiple causes > > + */ > > + if (bp->attr.bp_addr < TASK_SIZE) > > + rc = NOTIFY_DONE; > > Is that < TASK_SIZE an accurate check? We want support for > userspace breakpoints on perf tools later, and those don't want > signals. > Well, signal generation for user-space breakpoints happened unconditionally for 'historical' reasons (guess that Alan Stern's original patch had it that way). We could change that into a 'ptrace-only' signal generation now. > We do this cleanup in the beginning of the breakpoint handler: > > current->thread.debugreg6 &= ~DR_TRAP_BITS; > > And from ptrace.c:ptrace_triggered(): > > thread->debugreg6 |= (DR_TRAP0 << i); > > This is called on perf_bp_event(). > Instead of checking if this is a userspace thread, we should actually > check if this is a ptrace breakpoint by looking at this > in the end of hw_breakpoint_handler(). > > current->thread.debugreg6 & DR_TRAP_BITS > > Only ptrace breakpoints require signals. > Yes, this does look like a clean way to limit signals to those requests that are interested (I was looking at round-about ways like doing a lookup based on callback functions). I will send the next version of the patch with the above changes. Thanks, K.Prasad -- 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/