The memory-barriers.txt document contains an obsolete passage stating that
smp_read_barrier_depends() is required to force ordering for read-to-write
dependencies. We now know that this is not required, even for DEC Alpha.
This commit therefore updates this passage to state that read-to-write
dependencies are respected even without smp_read_barrier_depends().
Reported-by: Lance Roy <ldr...@gmail.com>
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Cc: David Howells <dhowe...@redhat.com>
Cc: Will Deacon <will.dea...@arm.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Jonathan Corbet <cor...@lwn.net>
Cc: Alan Stern <st...@rowland.harvard.edu>
Cc: Andrea Parri <parri.and...@gmail.com>
Cc: Jade Alglave <j.algl...@ucl.ac.uk>
Cc: Luc Maranget <luc.maran...@inria.fr>
diff --git a/Documentation/memory-barriers.txt
b/Documentation/memory-barriers.txt
index 9d5e0f853f08..a8a91b9d5a1b 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -594,7 +594,10 @@ between the address load and the data load:
This enforces the occurrence of one of the two implications, and prevents the
third possibility from arising.
-A data-dependency barrier must also order against dependent writes:
+A data-dependency barrier is not required to order dependent writes
+because the CPUs that the Linux kernel supports don't do writes until
+they are certain (1) that the write will actually happen, (2) of the
+location of the write, and (3) of the value to be written.
CPU 1 CPU 2
=============== ===============
@@ -603,19 +606,19 @@ A data-dependency barrier must also order against
dependent writes:
<write barrier>
WRITE_ONCE(P, &B);
Q = READ_ONCE(P);
- <data dependency barrier>
*Q = 5;
-The data-dependency barrier must order the read into Q with the store
-into *Q. This prohibits this outcome:
+Therefore, no data-dependency barrier is required to order the read into
+Q with the store into *Q. In other words, this outcome is prohibited,
+even without a data-dependency barrier:
(Q == &B) && (B == 4)
Please note that this pattern should be rare. After all, the whole point
of dependency ordering is to -prevent- writes to the data structure, along
with the expensive cache misses associated with those writes. This pattern
-can be used to record rare error conditions and the like, and the ordering
-prevents such records from being lost.
+can be used to record rare error conditions and the like, and the CPUs'
+naturally occurring ordering prevents such records from being lost.
[!] Note that this extremely counterintuitive situation arises most easily on
