From patchwork Fri May 12 21:07:09 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Thomas Gleixner X-Patchwork-Id: 13239719 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5C4CEC7EE25 for ; Fri, 12 May 2023 21:07:29 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.533914.830988 (Exim 4.92) (envelope-from ) id 1pxZyr-0006V6-1Z; Fri, 12 May 2023 21:07:13 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 533914.830988; Fri, 12 May 2023 21:07:13 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1pxZyq-0006Ui-UI; Fri, 12 May 2023 21:07:12 +0000 Received: by outflank-mailman (input) for mailman id 533914; Fri, 12 May 2023 21:07:11 +0000 Received: from se1-gles-sth1-in.inumbo.com ([159.253.27.254] helo=se1-gles-sth1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1pxZyo-0004FP-Vs for xen-devel@lists.xenproject.org; Fri, 12 May 2023 21:07:10 +0000 Received: from galois.linutronix.de (galois.linutronix.de [193.142.43.55]) by se1-gles-sth1.inumbo.com (Halon) with ESMTPS id f5c0cce5-f108-11ed-b229-6b7b168915f2; Fri, 12 May 2023 23:07:10 +0200 (CEST) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: f5c0cce5-f108-11ed-b229-6b7b168915f2 Message-ID: <20230512205255.875713771@linutronix.de> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1683925630; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: references:references; bh=CERacbFmAU0r2YyiKnQSCmUMCfPrzAHfru8m3ZNl0KY=; b=mlVKNpwrrZo8g1O5g2m+Me4aq4cJcRqHlfaggouQckkiLNuNl3gMI9wJGW+qTK77NJ8L1t BNZic2MIATVICLudtbI4AZFCsrK6yBKlMcV3RCwHbtApC9WMnfOTf/384jFhT6DDyDkJLU 68mjGx+kFnbdgLpBHRHDsZybKUUSd4DCRTEkmrpezylPkqkyKvPz75OmYdO3ljKUmH8DT+ qileb82DTQnWi+EWLiBSaCU365hMQBR6WvjZdiFAVtGRc9MA/Xyms7EJNSpFNCUC672GYD N9c4DHYS2yGrtOZTD5VUq08pLhp9Cqoy87Bcu1VdvrO1+17UGGzGP2jJGlQcVA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1683925630; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: references:references; bh=CERacbFmAU0r2YyiKnQSCmUMCfPrzAHfru8m3ZNl0KY=; b=pI9Kl0/aVsNuuAI/L8tX3o8uA++5XGmGpbq0v8JJXHVuwg59C4dNMi5XaYjRi8u2L2df6G eiU05lrMbsKzBPAA== From: Thomas Gleixner To: LKML Cc: x86@kernel.org, David Woodhouse , Andrew Cooper , Brian Gerst , Arjan van de Veen , Paolo Bonzini , Paul McKenney , Tom Lendacky , Sean Christopherson , Oleksandr Natalenko , Paul Menzel , "Guilherme G. Piccoli" , Piotr Gorski , Usama Arif , Juergen Gross , Boris Ostrovsky , xen-devel@lists.xenproject.org, Russell King , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, Catalin Marinas , Will Deacon , Guo Ren , linux-csky@vger.kernel.org, Thomas Bogendoerfer , linux-mips@vger.kernel.org, "James E.J. Bottomley" , Helge Deller , linux-parisc@vger.kernel.org, Paul Walmsley , Palmer Dabbelt , linux-riscv@lists.infradead.org, Mark Rutland , Sabin Rapan , "Michael Kelley (LINUX)" , Ross Philipson , Peter Zijlstra Subject: [patch V4 08/37] x86/smpboot: Remove unnecessary barrier() References: <20230512203426.452963764@linutronix.de> MIME-Version: 1.0 Date: Fri, 12 May 2023 23:07:09 +0200 (CEST) Peter stumbled over the barrier() after the invocation of smp_callin() in start_secondary(): "...this barrier() and it's comment seem weird vs smp_callin(). That function ends with an atomic bitop (it has to, at the very least it must not be weaker than store-release) but also has an explicit wmb() to order setup vs CPU_STARTING. There is no way the smp_processor_id() referred to in this comment can land before cpu_init() even without the barrier()." The barrier() along with the comment was added in 2003 with commit d8f19f2cac70 ("[PATCH] x86-64 merge") in the history tree. One of those well documented combo patches of that time which changes world and some more. The context back then was: /* * Dont put anything before smp_callin(), SMP * booting is too fragile that we want to limit the * things done here to the most necessary things. */ cpu_init(); smp_callin(); + /* otherwise gcc will move up smp_processor_id before the cpu_init */ + barrier(); Dprintk("cpu %d: waiting for commence\n", smp_processor_id()); Even back in 2003 the compiler was not allowed to reorder that smp_processor_id() invocation before the cpu_init() function call. Especially not as smp_processor_id() resolved to: asm volatile("movl %%gs:%c1,%0":"=r" (ret__):"i"(pda_offset(field)):"memory"); There is no trace of this change in any mailing list archive including the back then official x86_64 list discuss@x86-64.org, which would explain the problem this change solved. The debug prints are gone by now and the the only smp_processor_id() invocation today is farther down in start_secondary() after locking vector_lock which itself prevents reordering. Even if the compiler would be allowed to reorder this, the code would still be correct as GSBASE is set up early in the assembly code and is valid when the CPU reaches start_secondary(), while the code at the time when this barrier was added did the GSBASE setup in cpu_init(). As the barrier has zero value, remove it. Reported-by: Peter Zijlstra Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/20230509100421.GU83892@hirez.programming.kicks-ass.net --- V4: New patch --- arch/x86/kernel/smpboot.c | 2 -- 1 file changed, 2 deletions(-) --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -238,8 +238,6 @@ static void notrace start_secondary(void x86_cpuinit.early_percpu_clock_init(); smp_callin(); - /* otherwise gcc will move up smp_processor_id before the cpu_init */ - barrier(); /* Check TSC synchronization with the control CPU: */ check_tsc_sync_target();