From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751942Ab2JFGyq (ORCPT ); Sat, 6 Oct 2012 02:54:46 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:53846 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750760Ab2JFGyo (ORCPT ); Sat, 6 Oct 2012 02:54:44 -0400 Date: Sat, 6 Oct 2012 12:25:11 +0530 From: Srikar Dronamraju To: Oleg Nesterov Cc: Ingo Molnar , Peter Zijlstra , Ananth N Mavinakayanahalli , Anton Arapov , Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/4] uprobes: Kill set_swbp()->is_swbp_at_addr() Message-ID: <20121006065511.GC27781@linux.vnet.ibm.com> Reply-To: Srikar Dronamraju References: <20120923201921.GA27424@redhat.com> <20120923201945.GA27446@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20120923201945.GA27446@redhat.com> User-Agent: Mutt/1.5.20 (2009-06-14) x-cbid: 12100606-3534-0000-0000-00000D8050F3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Oleg Nesterov [2012-09-23 22:19:45]: > A separate patch for better documentation. > > set_swbp()->is_swbp_at_addr() is not needed for correctness, it is > harmless to do the unnecessary __replace_page(old_page, new_page) > when these 2 pages are identical. > > And it can not be counted as optimization. mmap/register races are > very unlikely, while in the likely case is_swbp_at_addr() adds the > extra get_user_pages() even if the caller is uprobe_mmap(current->mm) > and returns false. > > Note also that the semantics/usage of is_swbp_at_addr() in uprobe.c > is confusing. set_swbp() uses it to detect the case when this insn > was already modified by uprobes, that is why it should always compare > the opcode with UPROBE_SWBP_INSN even if the hardware (like powerpc) > has other trap insns. It doesn't matter if this "int3" was in fact > installed by gdb or application itself, we are going to "steal" this > breakpoint anyway and execute the original insn from vm_file even if > it no longer matches the memory. > > OTOH, handle_swbp()->find_active_uprobe() uses is_swbp_at_addr() to > figure out whether we need to send SIGTRAP or not if we can not find > uprobe, so in this case it should return true for all trap variants, > not only for UPROBE_SWBP_INSN. > > This patch removes set_swbp()->is_swbp_at_addr(), the next patches > will remove it from set_orig_insn() which is similar to set_swbp() > in this respect. So the only caller will be handle_swbp() and we > can make its semantics clear. > > Signed-off-by: Oleg Nesterov Acked-by: Srikar Dronamraju