This commit keeps only the historical and low-level discussion of smp_read_barrier_depends().
Signed-off-by: Paul E. McKenney <[email protected]> --- 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() + DATA DEPENDENCY READ_ONCE() All memory barriers except the data dependency barriers imply a compiler -- 2.5.2
