Feb. 25, 2013
7:15 a.m.
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