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| author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2015-04-20 09:09:27 -0400 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2015-05-27 15:56:17 -0400 |
| commit | cf9fbf8017e2ab5cb33b6602b626f7f005718124 (patch) | |
| tree | ca2ca92b0454c408c420fd9f744975eb08b3b9eb /Documentation/RCU/rcu_dereference.txt | |
| parent | 1ebee8017d84ec8a0ba893cf7b8be3f70ead088b (diff) | |
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 | |||
| 25 | for an example where the compiler can in fact deduce the exact | 25 | for an example where the compiler can in fact deduce the exact |
| 26 | value of the pointer, and thus cause misordering. | 26 | value of the pointer, and thus cause misordering. |
| 27 | 27 | ||
| 28 | o Do not use single-element RCU-protected arrays. The compiler | ||
| 29 | is within its right to assume that the value of an index into | ||
| 30 | such an array must necessarily evaluate to zero. The compiler | ||
| 31 | could then substitute the constant zero for the computation, so | ||
| 32 | that the array index no longer depended on the value returned | ||
| 33 | by rcu_dereference(). If the array index no longer depends | ||
| 34 | on rcu_dereference(), then both the compiler and the CPU | ||
| 35 | are within their rights to order the array access before the | ||
| 36 | rcu_dereference(), which can cause the array access to return | ||
| 37 | garbage. | ||
| 38 | |||
| 39 | o Avoid cancellation when using the "+" and "-" infix arithmetic | 28 | o Avoid cancellation when using the "+" and "-" infix arithmetic |
| 40 | operators. For example, for a given variable "x", avoid | 29 | operators. For example, for a given variable "x", avoid |
| 41 | "(x-x)". There are similar arithmetic pitfalls from other | 30 | "(x-x)". There are similar arithmetic pitfalls from other |
| @@ -76,14 +65,15 @@ o Do not use the results from the boolean "&&" and "||" when | |||
| 76 | dereferencing. For example, the following (rather improbable) | 65 | dereferencing. For example, the following (rather improbable) |
| 77 | code is buggy: | 66 | code is buggy: |
| 78 | 67 | ||
| 79 | int a[2]; | 68 | int *p; |
| 80 | int index; | 69 | int *q; |
| 81 | int force_zero_index = 1; | ||
| 82 | 70 | ||
| 83 | ... | 71 | ... |
| 84 | 72 | ||
| 85 | r1 = rcu_dereference(i1) | 73 | p = rcu_dereference(gp) |
| 86 | r2 = a[r1 && force_zero_index]; /* BUGGY!!! */ | 74 | q = &global_q; |
| 75 | q += p != &oom_p1 && p != &oom_p2; | ||
| 76 | r1 = *q; /* BUGGY!!! */ | ||
| 87 | 77 | ||
| 88 | The reason this is buggy is that "&&" and "||" are often compiled | 78 | The reason this is buggy is that "&&" and "||" are often compiled |
| 89 | using branches. While weak-memory machines such as ARM or PowerPC | 79 | using branches. While weak-memory machines such as ARM or PowerPC |
| @@ -94,14 +84,15 @@ o Do not use the results from relational operators ("==", "!=", | |||
| 94 | ">", ">=", "<", or "<=") when dereferencing. For example, | 84 | ">", ">=", "<", or "<=") when dereferencing. For example, |
| 95 | the following (quite strange) code is buggy: | 85 | the following (quite strange) code is buggy: |
| 96 | 86 | ||
| 97 | int a[2]; | 87 | int *p; |
| 98 | int index; | 88 | int *q; |
| 99 | int flip_index = 0; | ||
| 100 | 89 | ||
| 101 | ... | 90 | ... |
| 102 | 91 | ||
| 103 | r1 = rcu_dereference(i1) | 92 | p = rcu_dereference(gp) |
| 104 | r2 = a[r1 != flip_index]; /* BUGGY!!! */ | 93 | q = &global_q; |
| 94 | q += p > &oom_p; | ||
| 95 | r1 = *q; /* BUGGY!!! */ | ||
| 105 | 96 | ||
| 106 | As before, the reason this is buggy is that relational operators | 97 | As before, the reason this is buggy is that relational operators |
| 107 | are often compiled using branches. And as before, although | 98 | are often compiled using branches. And as before, although |