doc: Clarify historical disclaimers in memory-barriers.txt

This commit makes it clear that the reason that these sections are
historical is that smp_read_barrier_depends() is no more.  It also
removes the point about comparison operations, given that there are
other optimizations that can break address dependencies.

Suggested-by: Jonas Oberhauser <>
Signed-off-by: Paul E. McKenney <>
Cc: Alan Stern <>
Cc: Andrea Parri <>
Cc: Will Deacon <>
Cc: Peter Zijlstra <>
Cc: Boqun Feng <>
Cc: Nicholas Piggin <>
Cc: David Howells <>
Cc: Jade Alglave <>
Cc: Luc Maranget <>
Cc: Akira Yokosawa <>
Cc: Daniel Lustig <>
Cc: Joel Fernandes <>
Cc: Jonathan Corbet <>
Cc: <>
Cc: <>
Signed-off-by: Neeraj Upadhyay (AMD) <>
1 file changed