Commit 1566bf4b authored by Joel Fernandes (Google)'s avatar Joel Fernandes (Google) Committed by Paul E. McKenney
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docs: memory-barriers: Add note on compiler transformation and address deps 



The compiler has the ability to cause misordering by destroying
address-dependency barriers if comparison operations are used. Add a
note about this to memory-barriers.txt in the beginning of both the
historical address-dependency sections and point to rcu-dereference.rst
for more information.

Signed-off-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
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>
Reviewed-by: default avatarAndrea Parri <parri.andrea@gmail.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 0bb80ecc

Documentation/memory-barriers.txt

+7 −0
Original line number Diff line number Diff line
@@ -396,6 +396,10 @@ 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.



     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
@@ -556,6 +560,9 @@ 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.



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