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| -rw-r--r-- | Documentation/memory-barriers.txt | 26 |
1 files changed, 19 insertions, 7 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 6b25efd4558..9dde54c55b2 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -608,26 +608,30 @@ as follows: b = p; /* BUG: Compiler can reorder!!! */ do_something(); -The solution is again ACCESS_ONCE(), which preserves the ordering between -the load from variable 'a' and the store to variable 'b': +The solution is again ACCESS_ONCE() and barrier(), which preserves the +ordering between the load from variable 'a' and the store to variable 'b': q = ACCESS_ONCE(a); if (q) { + barrier(); ACCESS_ONCE(b) = p; do_something(); } else { + barrier(); ACCESS_ONCE(b) = p; do_something_else(); } -You could also use barrier() to prevent the compiler from moving -the stores to variable 'b', but barrier() would not prevent the -compiler from proving to itself that a==1 always, so ACCESS_ONCE() -is also needed. +The initial ACCESS_ONCE() is required to prevent the compiler from +proving the value of 'a', and the pair of barrier() invocations are +required to prevent the compiler from pulling the two identical stores +to 'b' out from the legs of the "if" statement. It is important to note that control dependencies absolutely require a a conditional. For example, the following "optimized" version of -the above example breaks ordering: +the above example breaks ordering, which is why the barrier() invocations +are absolutely required if you have identical stores in both legs of +the "if" statement: q = ACCESS_ONCE(a); ACCESS_ONCE(b) = p; /* BUG: No ordering vs. load from a!!! */ @@ -643,9 +647,11 @@ It is of course legal for the prior load to be part of the conditional, for example, as follows: if (ACCESS_ONCE(a) > 0) { + barrier(); ACCESS_ONCE(b) = q / 2; do_something(); } else { + barrier(); ACCESS_ONCE(b) = q / 3; do_something_else(); } @@ -659,9 +665,11 @@ the needed conditional. For example: q = ACCESS_ONCE(a); if (q % MAX) { + barrier(); ACCESS_ONCE(b) = p; do_something(); } else { + barrier(); ACCESS_ONCE(b) = p; do_something_else(); } @@ -723,6 +731,10 @@ In summary: use smb_rmb(), smp_wmb(), or, in the case of prior stores and later loads, smp_mb(). + (*) If both legs of the "if" statement begin with identical stores + to the same variable, a barrier() statement is required at the + beginning of each leg of the "if" statement. + (*) Control dependencies require at least one run-time conditional between the prior load and the subsequent store, and this conditional must involve the prior load. If the compiler |