Messages in this thread Patch in this message | | | Subject | Re: system gets stuck in a lock during boot | From | Steven Rostedt <> | Date | Wed, 07 Oct 2009 10:55:29 -0400 |
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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 <jbaron@redhat.com> > > > > > > > > > > > > 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 <rostedt@goodmis.org> > > > > > > > > > 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_<call> = { > > > * .name = "<call>", > > > * .system = "<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 > > > > >
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