On Sun, Feb 24, 2013 at 08:53:20PM +0800, Peter Teoh wrote:
> Another good article on atomicty and data sizes:
> 
> http://www.ibm.com/developerworks/library/pa-atom/
> 
> On Sun, Feb 24, 2013 at 8:50 PM, Peter Teoh <htmldeveloper@gmail.com> wrote:
> 
> > in simple terms, any operation, in terms assembly instructions, which can
> > be executed in ONE instruction, is "atomic", because, just like an atom, it
> > cannot be broken up into parts.   any instructions that is longer than one,
> > for eg, TWO instruction, is NOT atomic, because in BETWEEN the first and
> > 2nd instruction, something like an interrupt can come in, and affect the
> > values of the operand when it is passed from instruction one to second
> > instruction.  To save me from reiteration:
> >
> > http://www.ibm.com/developerworks/library/pa-dalign/ (search for
> > "atomicity").
> >
> > http://stackoverflow.com/questions/381244/purpose-of-memory-alignment
> >
> > http://lwn.net/Articles/260832/
> >
> > http://www.songho.ca/misc/alignment/dataalign.html
> >
> >
> > http://www.cis.upenn.edu/~palsetia/cit595s08/Lectures08/alignmentOrdering.pdf
> >
> > Essentially, atomicity and non-alignment become problematic when u tried
> > to to read using non-byte addressing mode with non-aligned address.
> >
> > On Sun, Feb 24, 2013 at 5:42 PM, Shraddha Kamat <sh2008ka@gmail.com>wrote:
> >
> >> what is the relation between atomic operations and memory alignment ?
> >>
> >> I read from UTLK that "an unaligned memory access is not atomic"
> >>
> >> please explain me , I am not able to get the relationship between
> >> memory alignment and atomicity of the operation.
It seems for this same reason, every objects of a struct page is organized as 
double word.
<snip from linux/mm_types.h>
 * The objects in struct page are organized in double word blocks in
 * order to allows us to use atomic double word operations on portions
 * of struct page. That is currently only used by slub but the arrangement
 * allows the use of atomic double word operations on the flags/mapping
 * and lru list pointers also.
<snip from linux/mm_types.h>

-Amit