lkml.org 
[lkml]   [2013]   [Nov]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: current_thread_info() not respecting program order with gcc 4.8.x
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 ;-)


\
 
 \ /
  Last update: 2013-11-19 19:01    [W:0.121 / U:0.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site