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| author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2015-04-20 06:09:27 -0700 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2015-05-27 12:56:17 -0700 |
| commit | cf9fbf8017e2ab5cb33b6602b626f7f005718124 (patch) | |
| tree | ca2ca92b0454c408c420fd9f744975eb08b3b9eb /Documentation/RCU/rcu_dereference.txt | |
| parent | 1ebee8017d84ec8a0ba893cf7b8be3f70ead088b (diff) | |
| download | linux-cf9fbf8017e2ab5cb33b6602b626f7f005718124.tar.bz2 | |
documentation: RCU-protected array indexes no longer supported
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'Documentation/RCU/rcu_dereference.txt')
| -rw-r--r-- | Documentation/RCU/rcu_dereference.txt | 33 |
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 |