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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-11-27 09:20:40 -0800
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-12-05 11:57:53 -0800
commit9ad3c143d7d6942c66f27bc6c18f5df638f70aff (patch)
treeb0b52f72cd5d03677b2d643feaaa7220aa109f29 /Documentation/RCU
parent98c1ec7cefaadbf65680d116c3d8612b93a841a0 (diff)
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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@linux.vnet.ibm.com> [ paulmck: Adjusted to allow for David Howells feedback on prior commit. ]
Diffstat (limited to 'Documentation/RCU')
-rw-r--r--Documentation/RCU/Design/Requirements/Requirements.html3
-rw-r--r--Documentation/RCU/rcu_dereference.txt6
-rw-r--r--Documentation/RCU/whatisRCU.txt3
3 files changed, 4 insertions, 8 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); \
})