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Subject[PATCH tip/core/rcu 17/21] doc: De-emphasize smp_read_barrier_depends
This commit keeps only the historical and low-level discussion of

Signed-off-by: Paul E. McKenney <>
Documentation/RCU/Design/Requirements/Requirements.html | 3 ++-
Documentation/RCU/rcu_dereference.txt | 6 +-----
Documentation/RCU/whatisRCU.txt | 3 +--
Documentation/circular-buffers.txt | 3 +--
Documentation/memory-barriers.txt | 7 ++++---
5 files changed, 9 insertions(+), 13 deletions(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 62e847bcdcdd..571c3d75510f 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -581,7 +581,8 @@ This guarantee was only partially premeditated.
DYNIX/ptx used an explicit memory barrier for publication, but had nothing
resembling <tt>rcu_dereference()</tt> for subscription, nor did it
have anything resembling the <tt>smp_read_barrier_depends()</tt>
-that was later subsumed into <tt>rcu_dereference()</tt>.
+that was later subsumed into <tt>rcu_dereference()</tt> and later
+still into <tt>READ_ONCE()</tt>.
The need for these operations made itself known quite suddenly at a
late-1990s meeting with the DEC Alpha architects, back in the days when
DEC was still a free-standing company.
diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt
index 1acb26b09b48..ab96227bad42 100644
--- a/Documentation/RCU/rcu_dereference.txt
+++ b/Documentation/RCU/rcu_dereference.txt
@@ -122,11 +122,7 @@ o Be very careful about comparing pointers obtained from
Note that if checks for being within an RCU read-side
critical section are not required and the pointer is never
dereferenced, rcu_access_pointer() should be used in place
- of rcu_dereference(). The rcu_access_pointer() primitive
- does not require an enclosing read-side critical section,
- and also omits the smp_read_barrier_depends() included in
- rcu_dereference(), which in turn should provide a small
- performance gain in some CPUs (e.g., the DEC Alpha).
+ of rcu_dereference().

o The comparison is against a pointer that references memory
that was initialized "a long time ago." The reason
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index df62466da4e0..a27fbfb0efb8 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -600,8 +600,7 @@ don't forget about them when submitting patches making use of RCU!]

#define rcu_dereference(p) \
({ \
- typeof(p) _________p1 = p; \
- smp_read_barrier_depends(); \
+ typeof(p) _________p1 = READ_ONCE(p); \
(_________p1); \

diff --git a/Documentation/circular-buffers.txt b/Documentation/circular-buffers.txt
index d4628174b7c5..53e51caa3347 100644
--- a/Documentation/circular-buffers.txt
+++ b/Documentation/circular-buffers.txt
@@ -220,8 +220,7 @@ before it writes the new tail pointer, which will erase the item.

Note the use of READ_ONCE() and smp_load_acquire() to read the
opposition index. This prevents the compiler from discarding and
-reloading its cached value - which some compilers will do across
-smp_read_barrier_depends(). This isn't strictly needed if you can
+reloading its cached value. This isn't strictly needed if you can
be sure that the opposition index will _only_ be used the once.
The smp_load_acquire() additionally forces the CPU to order against
subsequent memory references. Similarly, smp_store_release() is used
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 2269e58fa073..c21a9b25cacf 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -236,9 +236,10 @@ There are some minimal guarantees that may be expected of a CPU:
and always in that order. On most systems, smp_read_barrier_depends()
does nothing, but it is required for DEC Alpha, and is supplied by
READ_ONCE(). The READ_ONCE() is also required to prevent compiler
- mischief. Please note that you should normally use something
+ mischief. Please note that you should almost always use something
like READ_ONCE() or rcu_dereference() instead of open-coding
- smp_read_barrier_depends().
+ smp_read_barrier_depends(), the only exceptions being in DEC Alpha
+ architecture-specific code and in ACCESS_ONCE itself.

(*) Overlapping loads and stores within a particular CPU will appear to be
ordered within that CPU. This means that for:
@@ -1816,7 +1817,7 @@ The Linux kernel has eight basic CPU memory barriers:
GENERAL mb() smp_mb()
WRITE wmb() smp_wmb()
READ rmb() smp_rmb()
- DATA DEPENDENCY read_barrier_depends() smp_read_barrier_depends()

All memory barriers except the data dependency barriers imply a compiler
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