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-rw-r--r--Documentation/RCU/rcu_dereference.txt33
1 files changed, 12 insertions, 21 deletions
diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt
index ceb05da5a5ac..66864d2a7f60 100644
--- a/Documentation/RCU/rcu_dereference.txt
+++ b/Documentation/RCU/rcu_dereference.txt
@@ -25,17 +25,6 @@ o You must use one of the rcu_dereference() family of primitives
for an example where the compiler can in fact deduce the exact
value of the pointer, and thus cause misordering.
-o Do not use single-element RCU-protected arrays. The compiler
- is within its right to assume that the value of an index into
- such an array must necessarily evaluate to zero. The compiler
- could then substitute the constant zero for the computation, so
- that the array index no longer depended on the value returned
- by rcu_dereference(). If the array index no longer depends
- on rcu_dereference(), then both the compiler and the CPU
- are within their rights to order the array access before the
- rcu_dereference(), which can cause the array access to return
- garbage.
-
o Avoid cancellation when using the "+" and "-" infix arithmetic
operators. For example, for a given variable "x", avoid
"(x-x)". There are similar arithmetic pitfalls from other
@@ -76,14 +65,15 @@ o Do not use the results from the boolean "&&" and "||" when
dereferencing. For example, the following (rather improbable)
code is buggy:
- int a[2];
- int index;
- int force_zero_index = 1;
+ int *p;
+ int *q;
...
- r1 = rcu_dereference(i1)
- r2 = a[r1 && force_zero_index]; /* BUGGY!!! */
+ p = rcu_dereference(gp)
+ q = &global_q;
+ q += p != &oom_p1 && p != &oom_p2;
+ r1 = *q; /* BUGGY!!! */
The reason this is buggy is that "&&" and "||" are often compiled
using branches. While weak-memory machines such as ARM or PowerPC
@@ -94,14 +84,15 @@ o Do not use the results from relational operators ("==", "!=",
">", ">=", "<", or "<=") when dereferencing. For example,
the following (quite strange) code is buggy:
- int a[2];
- int index;
- int flip_index = 0;
+ int *p;
+ int *q;
...
- r1 = rcu_dereference(i1)
- r2 = a[r1 != flip_index]; /* BUGGY!!! */
+ p = rcu_dereference(gp)
+ q = &global_q;
+ q += p > &oom_p;
+ r1 = *q; /* BUGGY!!! */
As before, the reason this is buggy is that relational operators
are often compiled using branches. And as before, although