Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759448AbZJGO5b (ORCPT ); Wed, 7 Oct 2009 10:57:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759429AbZJGO5a (ORCPT ); Wed, 7 Oct 2009 10:57:30 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.122]:45211 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759395AbZJGO52 (ORCPT ); Wed, 7 Oct 2009 10:57:28 -0400 Subject: Re: system gets stuck in a lock during boot From: Steven Rostedt Reply-To: rostedt@goodmis.org To: Mathieu Desnoyers Cc: Jason Baron , Justin Mattock , Ingo Molnar , Peter Zijlstra , Li Zefan , Frederic Weisbecker , Linux Kernel Mailing List In-Reply-To: <20091007144058.GB6110@Krystal> References: <20090825085919.GB14003@elte.hu> <4A94803A.5060408@gmail.com> <20090826073351.GE23435@elte.hu> <4A9549E5.5020002@gmail.com> <20091002211211.GA2633@redhat.com> <1254792249.13160.213.camel@gandalf.stny.rr.com> <20091006203225.GC2631@redhat.com> <1254880921.1696.112.camel@gandalf.stny.rr.com> <20091007143023.GA2659@redhat.com> <20091007144058.GB6110@Krystal> Content-Type: text/plain Organization: Kihon Technologies Inc. Date: Wed, 07 Oct 2009 10:55:29 -0400 Message-Id: <1254927329.1696.150.camel@gandalf.stny.rr.com> Mime-Version: 1.0 X-Mailer: Evolution 2.26.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5795 Lines: 144 On Wed, 2009-10-07 at 10:40 -0400, Mathieu Desnoyers wrote: > * Jason Baron (jbaron@redhat.com) wrote: > > On Tue, Oct 06, 2009 at 10:02:01PM -0400, Steven Rostedt wrote: > > > > So the problem I'm seeing is an oops on boot caused by the call->system pointer > > > > deference in event_create_dir(). The 'call' variable is of type 'struct > > > > ftrace_event_call'. > > > > > > > > What's going on is that the 'struct ftrace_event_call' is of size 168 bytes > > > > (sizeof(struct ftrace_event_call)) = 168 = 0xA8. However, in memory the > > > > structures are 16-byte aligned. Thus, the stride for walking through the > > > > pointers needs to be 176 (0xB0), but instead its 168 causing the oops. > > > > > > > > I've only seen this issue while using gcc (GCC) 4.5.0 20090916, on a > > > > vanilla 2.6.31 kernel. > > > > > > > > That said, I'm not sure the compiler is doing the wrong thing here. The > > > > 'struct ftrace_event_call' contains an embedded 'struct list_head' which > > > > is 16 bytes. According to the gcc docs, the aligned attribute, 'specifies a > > > > minimum alignment for the variable or structure field, measured in bytes'. > > > > Thus, at least according to the docs, gcc can increase the alignment of the > > > > 'struct ftrace_event_call', from its original specification of 4, to 16. Even > > > > in the case where we are working corectly the structures are 8-byte aligned. > > > > > > > > Thus, I would reccommend the patch below as a preventive measure. Its > > > > the minimal patch I've found to resolve this issue. In general, if we > > > > are going to walk data structures embedded in a special elf section, I > > > > think the general rules needs to be to set the alignment to the power of > > > > two which is greater than or equal to the largest item in the structure. > > > > > > > > thanks, > > > > > > > > -Jason > > > > > > > > Signed-off-by: Jason Baron > > > > > > > > > > > > diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h > > > > index a81170d..7182f03 100644 > > > > --- a/include/linux/ftrace_event.h > > > > +++ b/include/linux/ftrace_event.h > > > > @@ -124,7 +124,10 @@ struct ftrace_event_call { > > > > atomic_t profile_count; > > > > int (*profile_enable)(struct ftrace_event_call *); > > > > void (*profile_disable)(struct ftrace_event_call *); > > > > -}; > > > > +} __attribute__((aligned(16))); > > > > + > > > > +/* Align to the largest field in the data structure: > > > > + * sizeof(struct list_head) = 16 */ > > > > > > Is this true for i386? > > > > > > I just tried this patch and it seems to work. Can you give it a try. > > > > > > Signed-off-by: Steven Rostedt > > > > > > > > > diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h > > > index 4ec5e67..044b70d 100644 > > > --- a/include/linux/ftrace_event.h > > > +++ b/include/linux/ftrace_event.h > > > @@ -133,7 +133,7 @@ struct ftrace_event_call { > > > atomic_t profile_count; > > > int (*profile_enable)(void); > > > void (*profile_disable)(void); > > > -}; > > > +} __attribute__((aligned(sizeof(struct list_head)))); > > I don't like that. > > Basically, the vmlinux.lds.h linker script must have alignment > statements before each section, which match the alignment of the section > structures. Failure to do so would put padding at the beginning of the > section, which is definitely not working at all. I don't see how we can > automatically pass sizeof(struct list_head) to a linker script :/ OK, what about __attribute__((aligned((BITS_PER_LONG/8)*2))) That should also work in the linker script as well. With the added comment: /* * We must aligned by the largest item in the structure. This happens * to be the list_head, which consists of two pointers. */ > > Mathieu > > > > > > > #define FTRACE_MAX_PROFILE_SIZE 2048 > > > > > > diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h > > > index cc0d966..31e7637 100644 > > > --- a/include/trace/ftrace.h > > > +++ b/include/trace/ftrace.h > > > @@ -501,7 +501,6 @@ static void ftrace_profile_disable_##call(void) \ > > > * } > > > * > > > * static struct ftrace_event_call __used > > > - * __attribute__((__aligned__(4))) > > > * __attribute__((section("_ftrace_events"))) event_ = { > > > * .name = "", > > > * .system = "", > > > @@ -619,7 +618,6 @@ static int ftrace_raw_init_event_##call(void) \ > > > } \ > > > \ > > > static struct ftrace_event_call __used \ > > > -__attribute__((__aligned__(4))) \ > > > __attribute__((section("_ftrace_events"))) event_##call = { \ > > > .name = #call, \ > > > .system = __stringify(TRACE_SYSTEM), \ > > > > > > > > > > indeed your patch works as well for me, its much cleaner! > > > > However, I want to make sure this fix is sufficient and is the best way to > > address this type of issue in general. For example, I know tracepoints are > > using the aligned attribute in all 3 places -> definition, usage, and linker > > alignment. (adding Mathieu to 'cc list). Is just the definition 'aligned' > > sufficient? Also, once we find a method for solving these issues in general, > > we need to review all users of this kind of technique to make sure they are > > consistent. I also think your patch above needs to add a comment to say what > > its doing. Yes, I forgot to add the comment. One really does belong there. -- Steve > > > > thanks, > > > > -Jason > > > > > -- 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/