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Subject[PATCH tip/core/rcu 3/4] documentation: RCU-protected array indexes no longer supported
Date
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/RCU/arrayRCU.txt | 20 ++++++++++++++++----
Documentation/RCU/lockdep.txt | 10 ----------
Documentation/RCU/rcu_dereference.txt | 33 ++++++++++++---------------------
Documentation/RCU/whatisRCU.txt | 2 --
4 files changed, 28 insertions(+), 37 deletions(-)

diff --git a/Documentation/RCU/arrayRCU.txt b/Documentation/RCU/arrayRCU.txt
index 453ebe6953ee..f05a9afb2c39 100644
--- a/Documentation/RCU/arrayRCU.txt
+++ b/Documentation/RCU/arrayRCU.txt
@@ -10,7 +10,19 @@ also be used to protect arrays. Three situations are as follows:

3. Resizeable Arrays

-Each of these situations are discussed below.
+Each of these three situations involves an RCU-protected pointer to an
+array that is separately indexed. It might be tempting to consider use
+of RCU to instead protect the index into an array, however, this use
+case is -not- supported. The problem with RCU-protected indexes into
+arrays is that compilers can play way too many optimization games with
+integers, which means that the rules governing handling of these indexes
+are far more trouble than they are worth. If RCU-protected indexes into
+arrays prove to be particularly valuable (which they have not thus far),
+explicit cooperation from the compiler will be required to permit them
+to be safely used.
+
+That aside, each of the three RCU-protected pointer situations are
+described in the following sections.


Situation 1: Hash Tables
@@ -36,9 +48,9 @@ Quick Quiz: Why is it so important that updates be rare when
Situation 3: Resizeable Arrays

Use of RCU for resizeable arrays is demonstrated by the grow_ary()
-function used by the System V IPC code. The array is used to map from
-semaphore, message-queue, and shared-memory IDs to the data structure
-that represents the corresponding IPC construct. The grow_ary()
+function formerly used by the System V IPC code. The array is used
+to map from semaphore, message-queue, and shared-memory IDs to the data
+structure that represents the corresponding IPC construct. The grow_ary()
function does not acquire any locks; instead its caller must hold the
ids->sem semaphore.

diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
index cd83d2348fef..da51d3068850 100644
--- a/Documentation/RCU/lockdep.txt
+++ b/Documentation/RCU/lockdep.txt
@@ -47,11 +47,6 @@ checking of rcu_dereference() primitives:
Use explicit check expression "c" along with
srcu_read_lock_held()(). This is useful in code that
is invoked by both SRCU readers and updaters.
- rcu_dereference_index_check(p, c):
- Use explicit check expression "c", but the caller
- must supply one of the rcu_read_lock_held() functions.
- This is useful in code that uses RCU-protected arrays
- that is invoked by both RCU readers and updaters.
rcu_dereference_raw(p):
Don't check. (Use sparingly, if at all.)
rcu_dereference_protected(p, c):
@@ -64,11 +59,6 @@ checking of rcu_dereference() primitives:
but retain the compiler constraints that prevent duplicating
or coalescsing. This is useful when when testing the
value of the pointer itself, for example, against NULL.
- rcu_access_index(idx):
- Return the value of the index and omit all barriers, but
- retain the compiler constraints that prevent duplicating
- or coalescsing. This is useful when when testing the
- value of the index itself, for example, against -1.

The rcu_dereference_check() check expression can be any boolean
expression, but would normally include a lockdep expression. However,
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
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 88dfce182f66..b201d4cd77f9 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -879,9 +879,7 @@ SRCU: Initialization/cleanup

All: lockdep-checked RCU-protected pointer access

- rcu_access_index
rcu_access_pointer
- rcu_dereference_index_check
rcu_dereference_raw
rcu_lockdep_assert
rcu_sleep_check
--
1.8.1.5


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