[lttng-dev] current_thread_info() not respecting program order with gcc 4.8.x

[Peter Zijlstra]

On Tue, Nov 19, 2013 at 05:02:20PM +0000, Mathieu Desnoyers wrote:
> Unfortunately I don't have a ARM cross-compiler setup ready. Nathan could test
> it for us though.
> 
> It might shuffle things around enough to work around the issue, but with the
> approach you propose, I would be concerned about the compiler being within
> its rights to reorder the code into the following sequence:
> 
> struct thread_info *ptra, *ptrb;
> 
> ptra = current_thread_info();
> /*
>  * each current_thread_info() would have a clobber on *sp, which orders
>  * those two wrt each other.
>   */
> ptrb = current_thread_info();
> 
> load from ptra->preempt_count;
> /*
>  * however, the following accesses that depend on ptra and ptrb could be
>  * reordered if the compiler has no way to know that ptra and ptrb are
>  * aliased.
>  */
> store to ptrb->preempt_count;
> 
> One question that might be worth asking: with the local register variable
> extension (http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Local-Reg-Vars.html#Local-Reg-Vars)
> (thanks to Jakub for the pointer), should the compiler consider two variables
> bound to the same register as being aliased or not ? AFAIU, local reg vars appear
> to be architecture-specific, so maybe there is something fishy on ARM ?

Might help if you ask where the GCC people are on CC ;-)