Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759334AbZJGOcY (ORCPT ); Wed, 7 Oct 2009 10:32:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759281AbZJGOcX (ORCPT ); Wed, 7 Oct 2009 10:32:23 -0400 Received: from mx1.redhat.com ([209.132.183.28]:7559 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759271AbZJGOcX (ORCPT ); Wed, 7 Oct 2009 10:32:23 -0400 Date: Wed, 7 Oct 2009 10:30:24 -0400 From: Jason Baron To: Steven Rostedt Cc: Justin Mattock , Ingo Molnar , Peter Zijlstra , Li Zefan , Frederic Weisbecker , Linux Kernel Mailing List , mathieu.desnoyers@polymtl.ca Subject: Re: system gets stuck in a lock during boot Message-ID: <20091007143023.GA2659@redhat.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1254880921.1696.112.camel@gandalf.stny.rr.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4511 Lines: 112 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)))); > > #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. 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/