Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755342AbbB0VMZ (ORCPT ); Fri, 27 Feb 2015 16:12:25 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51092 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753539AbbB0VMY (ORCPT ); Fri, 27 Feb 2015 16:12:24 -0500 Message-ID: <54F0DD9D.8040701@redhat.com> Date: Fri, 27 Feb 2015 22:11:57 +0100 From: Denys Vlasenko User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 MIME-Version: 1.0 To: Andy Lutomirski CC: Borislav Petkov , Rusty Russell , Konrad Rzeszutek Wilk , X86 ML , Boris Ostrovsky , Oleg Nesterov , "linux-kernel@vger.kernel.org" Subject: Re: [RFC 2/3] x86: Switch all C consumers of kernel_stack to this_cpu_sp0 References: <54F0979D.5060001@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3307 Lines: 80 On 02/27/2015 08:56 PM, Andy Lutomirski wrote: > On Feb 27, 2015 8:13 AM, "Denys Vlasenko" wrote: >> >> On 02/27/2015 01:07 AM, Andy Lutomirski wrote: >>> This will make modifying the semantics of kernel_stack easier. >>> >>> Cc: Konrad Rzeszutek Wilk >>> Cc: Boris Ostrovsky >>> Cc: Rusty Russell >>> Signed-off-by: Andy Lutomirski >>> --- >>> arch/x86/include/asm/thread_info.h | 3 +-- >>> arch/x86/kernel/traps.c | 2 +- >>> 2 files changed, 2 insertions(+), 3 deletions(-) >>> >>> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h >>> index e82e95abc92b..92549053d86d 100644 >>> --- a/arch/x86/include/asm/thread_info.h >>> +++ b/arch/x86/include/asm/thread_info.h >>> @@ -163,8 +163,7 @@ DECLARE_PER_CPU(unsigned long, kernel_stack); >>> static inline struct thread_info *current_thread_info(void) >>> { >>> struct thread_info *ti; >>> - ti = (void *)(this_cpu_read_stable(kernel_stack) + >>> - KERNEL_STACK_OFFSET - THREAD_SIZE); >>> + ti = (void *)(this_cpu_sp0() - THREAD_SIZE); >>> return ti; >>> } >>> >>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c >>> index c74f2f5652da..d287ea779728 100644 >>> --- a/arch/x86/kernel/traps.c >>> +++ b/arch/x86/kernel/traps.c >>> @@ -174,7 +174,7 @@ void ist_begin_non_atomic(struct pt_regs *regs) >>> * will catch asm bugs and any attempt to use ist_preempt_enable >>> * from double_fault. >>> */ >>> - BUG_ON(((current_stack_pointer() ^ this_cpu_read_stable(kernel_stack)) >>> + BUG_ON(((current_stack_pointer() ^ (this_cpu_sp0() - 1)) >>> & ~(THREAD_SIZE - 1)) != 0); >> >> While we are at it, I propose a more readable version of this check: >> >> BUG_ON(this_cpu_sp0() - current_stack_pointer() >= THREAD_SIZE); >> >> Yes, I am aware that it is not equivalent to the existing condition >> - it uses the fact that this_cpu_sp0(), previous check ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Oops... I meant to say "...the fact that this_cpu_sp0() points to the very top of the stack, previous check ..." >> wasn't making that assumption. But that assumption is true, >> so shouldn't be a problem. > > You're missing an absolute value in here, though. This isn't a check > for overflow; it's a check that we aren't on an IST or other per cpu > stack. Yes, that's exactly what the condition checks for. It reads "is current stack pointer below task's kernel stack by no more than THREAD_SIZE?" which is only possible if we are on task's kernel stack: If current_stack_pointer() is elsewhere, it is either (a) much smaller than this_cpu_sp0(), and BUG_ON condition obviously triggers; or (b) it is somewhere above this_cpu_sp0(), in which case subtraction overflows and condition triggers too. How would you write this condition so that it's easily readable? Evidently, my version isn't as readable sa I hoped... -- 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/