From: "Paul E. McKenney" <[email protected]> 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 <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]> Cc: Alan Stern <[email protected]> Cc: Andrea Parri <[email protected]> Cc: Will Deacon <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Boqun Feng <[email protected]> Cc: Nicholas Piggin <[email protected]> Cc: David Howells <[email protected]> Cc: Jade Alglave <[email protected]> Cc: Luc Maranget <[email protected]> Cc: Akira Yokosawa <[email protected]> Cc: Daniel Lustig <[email protected]> Cc: Joel Fernandes <[email protected]> Cc: Jonathan Corbet <[email protected]> Cc: <[email protected]> Cc: <[email protected]> Signed-off-by: Neeraj Upadhyay (AMD) <[email protected]> --- Documentation/memory-barriers.txt | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index d414e145f912..4202174a6262 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -396,10 +396,11 @@ Memory barriers come in four basic varieties: (2) Address-dependency barriers (historical). - [!] This section is marked as HISTORICAL: For more up-to-date - information, including how compiler transformations related to pointer - comparisons can sometimes cause problems, see - Documentation/RCU/rcu_dereference.rst. + [!] This section is marked as HISTORICAL: it covers the long-obsolete + smp_read_barrier_depends() macro, the semantics of which are now + implicit in all marked accesses. For more up-to-date information, + including how compiler transformations can sometimes break address + dependencies, see Documentation/RCU/rcu_dereference.rst. An address-dependency barrier is a weaker form of read barrier. In the case where two loads are performed such that the second depends on the @@ -560,9 +561,11 @@ There are certain things that the Linux kernel memory barriers do not guarantee: ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) ---------------------------------------- -[!] This section is marked as HISTORICAL: For more up-to-date information, -including how compiler transformations related to pointer comparisons can -sometimes cause problems, see Documentation/RCU/rcu_dereference.rst. +[!] This section is marked as HISTORICAL: it covers the long-obsolete +smp_read_barrier_depends() macro, the semantics of which are now implicit +in all marked accesses. For more up-to-date information, including +how compiler transformations can sometimes break address dependencies, +see Documentation/RCU/rcu_dereference.rst. As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for DEC Alpha, which means that about the only people who need to pay attention -- 2.40.1
