alignment faults in 3.6

On Thu, Oct 04, 2012 at 06:10:26PM -0500, Rob Herring wrote:

I would think the scheduling while atomic messages are harmless in this
case. However, in addition to spewing out BUG messages this commit also
seems to eventually cause a kernel panic in __napi_complete. That panic
seems to go away if I put barrier() between the 2 accesses above which
eliminates the alignment faults. I haven't figured that part out yet.

There's at least a couple of problems here:

This seems like an overly aggressive compiler optimization considering
unaligned accesses are not supported by ldm/stm.

The alignment fault handler should handle kernel address faults atomically.

This is bad news.  do_alignment() can be called in almost any kernel
context, and it must work.  die() and oops dumps - specifically dump_mem()
and dump_instr() will suffer from exactly the same problem.

Will, can you take a look please?