Commit-ID: 9ad3c143d7d6942c66f27bc6c18f5df638f70aff
Gitweb: https://git.kernel.org/tip/9ad3c143d7d6942c66f27bc6c18f5df638f70aff
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Mon, 27 Nov 2017 09:20:40 -0800
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue, 5 Dec 2017 11:57:53 -0800
doc: De-emphasize smp_read_barrier_depends
This commit keeps only the historical and low-level discussion of
smp_read_barrier_depends().
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
[ paulmck: Adjusted to allow for David Howells feedback on prior commit. ]
---
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, 10 insertions(+), 12 deletions(-)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 62e847b..571c3d7 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 1acb26b..ab96227 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 df62466..a27fbfb 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 d462817..53e51ca 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 13fd35b..a863009 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1818,7 +1818,7 @@ 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()
+ DATA DEPENDENCY READ_ONCE()
All memory barriers except the data dependency barriers imply a compiler
@@ -2867,7 +2867,10 @@ access depends on a read, not all do, so it may not be relied on.
Other CPUs may also have split caches, but must coordinate between the various
cachelets for normal memory accesses. The semantics of the Alpha removes the
-need for coordination in the absence of memory barriers.
+need for hardware coordination in the absence of memory barriers, which
+permitted Alpha to sport higher CPU clock rates back in the day. However,
+please note that smp_read_barrier_depends() should not be used except in
+Alpha arch-specific code and within the READ_ONCE() macro.
CACHE COHERENCY VS DMA
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