atomic operation in 32 bit but no in 64!?

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Hi,
	I was reading the code of include/linux/fs.h and saw a comment
	before i_size_read() that says:

	/*
	 * NOTE: in a 32bit arch with a preemptable kernel and an UP
	 * compile the i_size_read/write must be atomic with respect to
	 * the local cpu (unlike with preempt disabled), but they don't
	 * need to be atomic with respect to other cpus like in true SMP
	 * (so they need either to either locally disable irq around the
	 * read or for example on x86 they can be still implemented as a
	 * cmpxchg8b without the need of the lock prefix). For SMP
	 * compiles and 64bit archs it makes no difference if preempt is
	 * enabled or not.
	 */

	 I don't understand why this funcion shouldn't be atomic in a 64
	 bit arch or why it isn't locked. Where is the race condition
	 prevented?

	 regards,
	 	diegows
-- 

--------------
Diego Woitasen

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