Re: system gets stuck in a lock during boot

[Justin P. Mattock]

Steven Rostedt wrote:
> On Tue, 2009-10-06 at 16:32 -0400, Jason Baron 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@xxxxxxxxxx>
> >
> >
> > 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@xxxxxxxxxxx>
>
>
> 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_<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),		\
>
>
>
>    
o.k. applied your patch, but unfortunantly
I still am hitting this kernel panic.

must admit I have no idea why this is doing this.
(but am willing to sit through this, because eventually
sooner or later will hit this if I update gcc).

Justin P. Mattock
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