From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-x444.google.com (mail-pf1-x444.google.com [IPv6:2607:f8b0:4864:20::444]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 41zP5p2RcSzDqSL for ; Mon, 27 Aug 2018 17:43:57 +1000 (AEST) Received: by mail-pf1-x444.google.com with SMTP id s13-v6so7185606pfi.7 for ; Mon, 27 Aug 2018 00:43:57 -0700 (PDT) Date: Mon, 27 Aug 2018 17:43:43 +1000 From: Nicholas Piggin To: Linus Torvalds Cc: Peter Anvin , Helge Deller , Nick Desaulniers , linux-alpha@vger.kernel.org, Linux Kernel Mailing List , arcml , linux-arm-kernel , linux-c6x-dev@linux-c6x.org, "moderated list:H8/300 ARCHITECTURE" , linux-hexagon@vger.kernel.org, "linux-ia64@vger.kernel.org" , "Linux/m68k" , linux-mips , "moderated list:NIOS2 ARCHITECTURE" , openrisc@lists.librecores.org, Parisc List , ppc-dev , linux-riscv@lists.infradead.org, linux-s390 , Linux-sh list , sparclinux , linux-um@lists.infradead.org Subject: Re: [PATCH] treewide: remove current_text_addr Message-ID: <20180827161121.07aa9da6@roar.ozlabs.ibm.com> In-Reply-To: References: <20180821202900.208417-1-ndesaulniers@google.com> <207784db-4fcc-85e7-a0b2-fec26b7dab81@gmx.de> <81141365-8168-799b-f34f-da5f92efaaf9@zytor.com> <7f49eeab-a5cc-867f-58fb-abd266f9c2c9@zytor.com> <6ca8a1d3-ff95-e9f4-f003-0a5af85bcb6f@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , [ Trimmed the cc list because my SMTP didn't accept that many addresses. ] On Sun, 26 Aug 2018 13:25:14 -0700 Linus Torvalds wrote: > On Sun, Aug 26, 2018 at 12:32 PM H. Peter Anvin wrote: > > > > Here is a full-blown (user space) test program demonstrating the whole > > technique and how to use it. > > So while I agree that some _THIS_IP_ users might be better off being > converted to __builtin_return_address(0) at the caller, I also think > that the whole "notailcall" thing shows why that can easily be more > problematic than just our currnet _THIS_IP_ solution. > > Honestly, I'd suggest: > > - just do the current_text_addr() to _THIS_IP_ conversion > > - keep _THIS_IP_ and make it be the generic one, and screw the whole > "some architectures might implement is better" issue. Nobody cares. > > - try to convince people to move away from the "we want the kernel > instruction pointer for the call" model entirely, and consider this a > "legacy" issue. > > The whole instruction pointer is a nasty thing. We should discourage > it and not make complex infrastructure for it. > > Instead, maybe we could encourage something like > > struct kernel_loc { const char *file; const char *fn; int line; }; > > #define __GEN_LOC__(n) \ > ({ static const struct kernel_loc n = { \ > __FILE__, __FUNCTION__, __LINE__ \ > }; &n; }) > > #define _THIS_LOC_ __GEN_LOC__(__UNIQUE_ID(loc)) > > which is a hell of a lot nicer to use, and actually allows gcc to > optimize things (try it: if you pass a _THIS_LOC_ off to an inline > function, and that inline function uses the name and line number, gcc > will pick them up directly, without the extra structure dereference. > > Wouldn't it be much nicer to pass these kinds of "location pointer" > around, rather than the nasty _THIS_IP_ thing? Seems nice. Do you even need this unique ID thing? AFAIKS the name would never really be useful. It could perhaps go into a cold data section too, I assume the common case is that you do not access it. Although gcc will end up putting the file and function names into regular rodata. Possibly we could add a printk specifier for it, pass it through to existing BUG, etc macros that want exactly this, etc. Makes a lot of sense. Thanks, Nick