diff options
Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 8 |
1 files changed, 4 insertions, 4 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 7146da061693..e1c355e84edd 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -185,7 +185,7 @@ As a further example, consider this sequence of events: =============== =============== { A == 1, B == 2, C == 3, P == &A, Q == &C } B = 4; Q = P; - P = &B D = *Q; + P = &B; D = *Q; There is an obvious data dependency here, as the value loaded into D depends on the address retrieved from P by CPU 2. At the end of the sequence, any of the @@ -569,7 +569,7 @@ following sequence of events: { A == 1, B == 2, C == 3, P == &A, Q == &C } B = 4; <write barrier> - WRITE_ONCE(P, &B) + WRITE_ONCE(P, &B); Q = READ_ONCE(P); D = *Q; @@ -1721,7 +1721,7 @@ of optimizations: and WRITE_ONCE() are more selective: With READ_ONCE() and WRITE_ONCE(), the compiler need only forget the contents of the indicated memory locations, while with barrier() the compiler must - discard the value of all memory locations that it has currented + discard the value of all memory locations that it has currently cached in any machine registers. Of course, the compiler must also respect the order in which the READ_ONCE()s and WRITE_ONCE()s occur, though the CPU of course need not do so. @@ -1833,7 +1833,7 @@ Aside: In the case of data dependencies, the compiler would be expected to issue the loads in the correct order (eg. `a[b]` would have to load the value of b before loading a[b]), however there is no guarantee in the C specification that the compiler may not speculate the value of b -(eg. is equal to 1) and load a before b (eg. tmp = a[1]; if (b != 1) +(eg. is equal to 1) and load a[b] before b (eg. tmp = a[1]; if (b != 1) tmp = a[b]; ). There is also the problem of a compiler reloading b after having loaded a[b], thus having a newer copy of b than a[b]. A consensus has not yet been reached about these problems, however the READ_ONCE() |