Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752251AbbGIAhK (ORCPT ); Wed, 8 Jul 2015 20:37:10 -0400 Received: from mail-la0-f52.google.com ([209.85.215.52]:34257 "EHLO mail-la0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751007AbbGIAhF (ORCPT ); Wed, 8 Jul 2015 20:37:05 -0400 MIME-Version: 1.0 In-Reply-To: <559D8250.8000707@gmail.com> References: <20150708160750.GQ19282@twins.programming.kicks-ass.net> <559D8250.8000707@gmail.com> From: Andy Lutomirski Date: Wed, 8 Jul 2015 17:36:43 -0700 Message-ID: Subject: Re: Kernel broken on processors without performance counters To: Jason Baron Cc: Peter Zijlstra , Mikulas Patocka , Paul Mackerras , Arnaldo Carvalho de Melo , Kees Cook , Andrea Arcangeli , Vince Weaver , "hillf.zj" , Valdis Kletnieks , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3286 Lines: 83 On Wed, Jul 8, 2015 at 1:04 PM, Jason Baron wrote: > On 07/08/2015 01:37 PM, Andy Lutomirski wrote: >> On Wed, Jul 8, 2015 at 9:07 AM, Peter Zijlstra wrote: >>> On Wed, Jul 08, 2015 at 11:17:38AM -0400, Mikulas Patocka wrote: >>>> Hi >>>> >>>> I found out that the patch a66734297f78707ce39d756b656bfae861d53f62 breaks >>>> the kernel on processors without performance counters, such as AMD K6-3. >>>> Reverting the patch fixes the problem. >>>> >>>> The static key rdpmc_always_available somehow gets set (I couldn't really >>>> find out what is setting it, the function set_attr_rdpmc is not executed), >>>> cr4_set_bits(X86_CR4_PCE) is executed and that results in a crash on boot >>>> when attempting to execute init, because the proecssor doesn't support >>>> that bit in CR4. >>> Urgh, the static key trainwreck bites again. >>> >>> One is not supposed to mix static_key_true() and STATIC_KEY_INIT_FALSE. >>> >>> Does this make it go again? >>> >>> --- >>> arch/x86/include/asm/mmu_context.h | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h >>> index 5e8daee7c5c9..804a3a6030ca 100644 >>> --- a/arch/x86/include/asm/mmu_context.h >>> +++ b/arch/x86/include/asm/mmu_context.h >>> @@ -23,7 +23,7 @@ extern struct static_key rdpmc_always_available; >>> >>> static inline void load_mm_cr4(struct mm_struct *mm) >>> { >>> - if (static_key_true(&rdpmc_always_available) || >>> + if (static_key_false(&rdpmc_always_available) || >> In what universe is "static_key_false" a reasonable name for a >> function that returns true if a static key is true? >> >> Can we rename that function? And could we maybe make static keys type >> safe? I.e. there would be a type that starts out true and a type that >> starts out false. > > So the 'static_key_false' is really branch is initially false. We had > a naming discussion before, but if ppl think its confusing, > 'static_key_init_false', or 'static_key_default_false' might be better, > or other ideas.... I agree its confusing. I think the current naming is almost maximally bad. The naming would be less critical if it were typesafe, though. > > In terms of getting the type to match so we don't have these > mismatches, I think we could introduce 'struct static_key_false' > and 'struct static_key_true' with proper initializers. However, > 'static_key_slow_inc()/dec()' would also have to add the > true/false modifier. I think that would be okay. > Or maybe we do: > > struct static_key_false { > struct static_key key; > } random_key; > > and then the 'static_key_sloc_inc()/dec()' would just take > a &random_key.key.... That would be okay, too. Or it could be a macro that has the same effect. > > If we were to change this, I don't think it would be too hard to > introduce the new API, convert subsystems over time and then > drop the old one. And we might discover a bug or three while doing it :) --Andy -- 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/