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 rcu_dereference() for subscription, nor did it have anything resembling the smp_read_barrier_depends() -that was later subsumed into rcu_dereference(). +that was later subsumed into rcu_dereference() and later +still into READ_ONCE(). 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 13fd35b6a597..a863009849a3 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