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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 <jonas.oberhauser@huaweicloud.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: <linux-arch@vger.kernel.org>
Cc: <linux-doc@vger.kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>
This commit is contained in:
Paul E. McKenney 2023-10-20 11:04:27 -07:00 committed by Neeraj Upadhyay (AMD)
parent c49956be75
commit ad94463029
1 changed files with 10 additions and 7 deletions
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17
Documentation/memory-barriers.txt
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@ -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