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author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2017-10-09 19:15:21 +0300 |
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committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2017-12-04 21:50:08 +0300 |
commit | 40555946447a394889243e4393e312f65d847e1e (patch) | |
tree | e34032c0d1276b86076df9f0ad674b8850a4df08 | |
parent | 4fbd8d194f06c8a3fd2af1ce560ddb31f7ec8323 (diff) | |
download | linux-40555946447a394889243e4393e312f65d847e1e.tar.xz |
doc: READ_ONCE() now implies smp_barrier_depends()
This commit updates an example in memory-barriers.txt to account for
the fact that READ_ONCE() now implies smp_barrier_depends().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Added MEMORY_BARRIER instructions from DEC Alpha from
READ_ONCE(), per David Howells's feedback. ]
-rw-r--r-- | Documentation/memory-barriers.txt | 15 |
1 files changed, 9 insertions, 6 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 479ecec80593..13fd35b6a597 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -227,17 +227,20 @@ There are some minimal guarantees that may be expected of a CPU: (*) On any given CPU, dependent memory accesses will be issued in order, with respect to itself. This means that for: - Q = READ_ONCE(P); smp_read_barrier_depends(); D = READ_ONCE(*Q); + Q = READ_ONCE(P); D = READ_ONCE(*Q); the CPU will issue the following memory operations: Q = LOAD P, D = LOAD *Q - and always in that order. On most systems, smp_read_barrier_depends() - does nothing, but it is required for DEC Alpha. The READ_ONCE() - is required to prevent compiler mischief. Please note that you - should normally use something like rcu_dereference() instead of - open-coding smp_read_barrier_depends(). + and always in that order. However, on DEC Alpha, READ_ONCE() also + emits a memory-barrier instruction, so that a DEC Alpha CPU will + instead issue the following memory operations: + + Q = LOAD P, MEMORY_BARRIER, D = LOAD *Q, MEMORY_BARRIER + + Whether on DEC Alpha or not, the READ_ONCE() also prevents compiler + mischief. (*) Overlapping loads and stores within a particular CPU will appear to be ordered within that CPU. This means that for: |