Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753803AbZJZQ5O (ORCPT ); Mon, 26 Oct 2009 12:57:14 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753591AbZJZQ5N (ORCPT ); Mon, 26 Oct 2009 12:57:13 -0400 Received: from mail-ew0-f208.google.com ([209.85.219.208]:48927 "EHLO mail-ew0-f208.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753480AbZJZQ5N (ORCPT ); Mon, 26 Oct 2009 12:57:13 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:reply-to:to:cc:in-reply-to:references:content-type :organization:date:message-id:mime-version:x-mailer :content-transfer-encoding; b=Yzn1WC/bjAmGZnQu/oeM6sA8sjhMglM6tfE3DsybPO1d4zne7QR+2OdKgmQ/1IFWCP soDAuCyPBZYrRi5cUXOvS/PWj8xI5ocropI4FJP4Qj0lT9HppLw1Dck5o8volQeeC17z EvetXueLF10oO+8OZhP6k7HvK8Dl7lc0KWBLI= Subject: Re: [PATCH -v5 10/11] tracing: add function graph tracer support for MIPS From: Wu Zhangjin Reply-To: wuzhangjin@gmail.com To: rostedt@goodmis.org Cc: linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Ralf Baechle , Nicholas Mc Guire , Richard Sandiford , David Daney , Adam Nemet , Patrik Kluba In-Reply-To: <1256574775.26028.321.camel@gandalf.stny.rr.com> References: <028867b99ec532b84963a35e7d552becc783cafc.1256483735.git.wuzhangjin@gmail.com> <2f73eae542c47ac5bbb9f7280e6c0271d193e90d.1256483735.git.wuzhangjin@gmail.com> <3e0c2d7d8b8f196a8153beb41ea7f3cbf42b3d84.1256483735.git.wuzhangjin@gmail.com> <54c417629e91f40b2bbb4e08cda2a4e6527824c0.1256483735.git.wuzhangjin@gmail.com> <29bccff04932e993ecd9f516d8b6dcf84e2ceecf.1256483735.git.wuzhangjin@gmail.com> <72f2270f7b6e01ca7a4cdf4ac8c21778e5d9652f.1256483735.git.wuzhangjin@gmail.com> <6140dd8f4e1783e5ac30977cf008bb98e4698322.1256483735.git.wuzhangjin@gmail.com> <49b3c441a57f4db423732f81432a3450ccb3240e.1256483735.git.wuzhangjin@gmail.com> <6ad82af0c2ec8ef7b9f536b0a97bf65d385c3945.1256483735.git.wuzhangjin@gmail.com> <1256570001.26028.298.camel@gandalf.stny.rr.com> <1256573467.5642.214.camel@falcon> <1256574775.26028.321.camel@gandalf.stny.rr.com> Content-Type: text/plain Organization: DSLab, Lanzhou University, China Date: Tue, 27 Oct 2009 00:57:05 +0800 Message-Id: <1256576225.5642.244.camel@falcon> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3585 Lines: 107 Hi, [...] > > Yeah, and probably not as important in the mips world, as it is used > more with embedded devices than desktops. We must always take the > "paranoid" approach for tracing. At least in PPC and x86, we assume > everything is broken ;-) And we want to be as robust as possible. If > something goes wrong, we want to detect it ASAP and report it. And keep > the system from crashing. > > At least with MIPS we don't need to worry about crashing Linus's > desktop. With is the #1 priority we have on x86 ... "Don't crash Linus's > desktop!". > > If Linus sees a warning, he'll bitch at us. If we crash his box, and he > was to lose any information, he'll strip out our code! > Okay, a new patch for all of the exception handling will go into -v7. > > > > > So, we just need to replace this: > > > > if ((code & MOV_FP_SP) == MOV_FP_SP) > > return parent_addr; > > > > by > > > > #define S_INSN (0xafb0 << 16) > > > > if ((code & S_INSN) != S_INSN) > > return parent_addr; > > I would be even more paranoid, and make sure each of those stores, store > into sp. get it :-) (I need to be more paranoid too, otherwise, Steven will not accept my patches!) > > > > > > > > > > + > > > > + sp = fp + (code & STACK_OFFSET_MASK); > > > > + ra = *(unsigned long *)sp; > > > > > > Also might want to make the above into a asm with exception handling. > > > > > > > + > > > > + if (ra == parent) > > > > + return sp; > > > > + > > > > + ftrace_graph_stop(); > > > > + WARN_ON(1); > > > > + return parent_addr; > > > > > > Hmm, may need to do more than this. See below. > > > [...] > > > > + > > > > + old = *parent; > > > > + > > > > + parent = (unsigned long *)ftrace_get_parent_addr(self_addr, old, > > > > + (unsigned long)parent, > > > > + fp); > > > > + > > > > + *parent = return_hooker; > > > > > > Although you may have turned off fgraph tracer in > > > ftrace_get_parent_addr, nothing stops the below from messing with the > > > stack. The return stack may get off sync and break later. If you fail > > > the above, you should not be calling the push function below. > > > > > > > We need to really stop before ftrace_push_return_trace to avoid messing > > with the stack :-) but if we have stopped the tracer, is it important to > > mess with the stack or not? > > The ftrace_push_return_trace does not test if the trace stopped, that is > expected to be done by the caller. If you mess with the stack set up, > you will crash the box. Remember, before the failure, you could have > already replaced return jumps. Those will still be falling back to the > return_to_handler. If you mess with the stack, but don't update the > return, the other returns will be out of sync and call the wrong return > address. > As you can see, after stopping the function graph tracer(here the function is non-leaf) with ftrace_graph_stop() in ftrace_get_parent_addr(), I return the old parent_addr, this is only the stack address in the stack space of ftrace_graph_caller, which means that, I never touch the real stack address of the non-leaf function, and it will not trap into the return_to_handler hooker 'Cause the non-leaf function will load it's own normal return address from it's own stack, and then just return back normally. -- This is another trick :-) Regards, Wu Zhangjin -- 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/