On Sun, 24 Feb 2013 11:50:14 +0100, richard -rw- weinberger said: > On Sun, Feb 24, 2013 at 10:42 AM, Shraddha Kamat <sh200...@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. > > Not all CPUs support unaligned memory access, such an access may cause a fault > which needs to be fixed by the kernel...
There's a more subtle issue - an unaligned access can be split across a cache line boundary, requiring 2 separate memory accesses to do the read or write. This can result in CPU A fetching the first half of the variable, CPU B updating both halves, and then A fetching the second half of the now updated variable.. This can bite you even on CPUs that support unaligned accesses.
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