From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752649Ab2IXJEG (ORCPT ); Mon, 24 Sep 2012 05:04:06 -0400 Received: from merlin.infradead.org ([205.233.59.134]:41329 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751635Ab2IXJEE convert rfc822-to-8bit (ORCPT ); Mon, 24 Sep 2012 05:04:04 -0400 Message-ID: <1348477426.11847.6.camel@twins> Subject: Re: [PATCH 1/4] uprobes: Kill set_swbp()->is_swbp_at_addr() From: Peter Zijlstra To: Oleg Nesterov Cc: Ingo Molnar , Srikar Dronamraju , Ananth N Mavinakayanahalli , Anton Arapov , Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org Date: Mon, 24 Sep 2012 11:03:46 +0200 In-Reply-To: <20120923201945.GA27446@redhat.com> References: <20120923201945.GA27446@redhat.com> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.2- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 2012-09-23 at 22:19 +0200, Oleg Nesterov wrote: > 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. It does save a page of memory though... > 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. This does leave me with the question of _why_ you're removing it.. the above says what it does, and maybe gives a clue as to why you think it is superfluous but I think its better to be clear on this.