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Subject[PATCH tip/core/rcu 4/7] doc: Convert to rcu_dereference.txt to rcu_dereference.rst
Date
From: Amol Grover <frextrite@gmail.com>

This patch converts rcu_dereference.txt to rcu_dereference.rst and
adds it to index.rst

Signed-off-by: Amol Grover <frextrite@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
Documentation/RCU/index.rst | 1 +
.../{rcu_dereference.txt => rcu_dereference.rst} | 75 ++++++++++++----------
2 files changed, 42 insertions(+), 34 deletions(-)
rename Documentation/RCU/{rcu_dereference.txt => rcu_dereference.rst} (88%)

diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
index b9b1148..c81d0e4 100644
--- a/Documentation/RCU/index.rst
+++ b/Documentation/RCU/index.rst
@@ -8,6 +8,7 @@ RCU concepts
:maxdepth: 3

arrayRCU
+ rcu_dereference
whatisRCU
rcu
listRCU
diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.rst
similarity index 88%
rename from Documentation/RCU/rcu_dereference.txt
rename to Documentation/RCU/rcu_dereference.rst
index bf699e8..c9667eb 100644
--- a/Documentation/RCU/rcu_dereference.txt
+++ b/Documentation/RCU/rcu_dereference.rst
@@ -1,4 +1,7 @@
+.. _rcu_dereference_doc:
+
PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()
+===============================================================

Most of the time, you can use values from rcu_dereference() or one of
the similar primitives without worries. Dereferencing (prefix "*"),
@@ -8,7 +11,7 @@ subtraction of constants, and casts all work quite naturally and safely.
It is nevertheless possible to get into trouble with other operations.
Follow these rules to keep your RCU code working properly:

-o You must use one of the rcu_dereference() family of primitives
+- You must use one of the rcu_dereference() family of primitives
to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU
will complain. Worse yet, your code can see random memory-corruption
bugs due to games that compilers and DEC Alpha can play.
@@ -25,24 +28,24 @@ 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 You are only permitted to use rcu_dereference on pointer values.
+- You are only permitted to use rcu_dereference on pointer values.
The compiler simply knows too much about integral values to
trust it to carry dependencies through integer operations.
There are a very few exceptions, namely that you can temporarily
cast the pointer to uintptr_t in order to:

- o Set bits and clear bits down in the must-be-zero low-order
+ - Set bits and clear bits down in the must-be-zero low-order
bits of that pointer. This clearly means that the pointer
must have alignment constraints, for example, this does
-not- work in general for char* pointers.

- o XOR bits to translate pointers, as is done in some
+ - XOR bits to translate pointers, as is done in some
classic buddy-allocator algorithms.

It is important to cast the value back to pointer before
doing much of anything else with it.

-o Avoid cancellation when using the "+" and "-" infix arithmetic
+- Avoid cancellation when using the "+" and "-" infix arithmetic
operators. For example, for a given variable "x", avoid
"(x-(uintptr_t)x)" for char* pointers. The compiler is within its
rights to substitute zero for this sort of expression, so that
@@ -54,16 +57,16 @@ o Avoid cancellation when using the "+" and "-" infix arithmetic
"p+a-b" is safe because its value still necessarily depends on
the rcu_dereference(), thus maintaining proper ordering.

-o If you are using RCU to protect JITed functions, so that the
+- If you are using RCU to protect JITed functions, so that the
"()" function-invocation operator is applied to a value obtained
(directly or indirectly) from rcu_dereference(), you may need to
interact directly with the hardware to flush instruction caches.
This issue arises on some systems when a newly JITed function is
using the same memory that was used by an earlier JITed function.

-o Do not use the results from relational operators ("==", "!=",
+- Do not use the results from relational operators ("==", "!=",
">", ">=", "<", or "<=") when dereferencing. For example,
- the following (quite strange) code is buggy:
+ the following (quite strange) code is buggy::

int *p;
int *q;
@@ -81,11 +84,11 @@ o Do not use the results from relational operators ("==", "!=",
after such branches, but can speculate loads, which can again
result in misordering bugs.

-o Be very careful about comparing pointers obtained from
+- Be very careful about comparing pointers obtained from
rcu_dereference() against non-NULL values. As Linus Torvalds
explained, if the two pointers are equal, the compiler could
substitute the pointer you are comparing against for the pointer
- obtained from rcu_dereference(). For example:
+ obtained from rcu_dereference(). For example::

p = rcu_dereference(gp);
if (p == &default_struct)
@@ -93,7 +96,7 @@ o Be very careful about comparing pointers obtained from

Because the compiler now knows that the value of "p" is exactly
the address of the variable "default_struct", it is free to
- transform this code into the following:
+ transform this code into the following::

p = rcu_dereference(gp);
if (p == &default_struct)
@@ -105,14 +108,14 @@ o Be very careful about comparing pointers obtained from

However, comparisons are OK in the following cases:

- o The comparison was against the NULL pointer. If the
+ - The comparison was against the NULL pointer. If the
compiler knows that the pointer is NULL, you had better
not be dereferencing it anyway. If the comparison is
non-equal, the compiler is none the wiser. Therefore,
it is safe to compare pointers from rcu_dereference()
against NULL pointers.

- o The pointer is never dereferenced after being compared.
+ - The pointer is never dereferenced after being compared.
Since there are no subsequent dereferences, the compiler
cannot use anything it learned from the comparison
to reorder the non-existent subsequent dereferences.
@@ -124,31 +127,31 @@ o Be very careful about comparing pointers obtained from
dereferenced, rcu_access_pointer() should be used in place
of rcu_dereference().

- o The comparison is against a pointer that references memory
+ - The comparison is against a pointer that references memory
that was initialized "a long time ago." The reason
this is safe is that even if misordering occurs, the
misordering will not affect the accesses that follow
the comparison. So exactly how long ago is "a long
time ago"? Here are some possibilities:

- o Compile time.
+ - Compile time.

- o Boot time.
+ - Boot time.

- o Module-init time for module code.
+ - Module-init time for module code.

- o Prior to kthread creation for kthread code.
+ - Prior to kthread creation for kthread code.

- o During some prior acquisition of the lock that
+ - During some prior acquisition of the lock that
we now hold.

- o Before mod_timer() time for a timer handler.
+ - Before mod_timer() time for a timer handler.

There are many other possibilities involving the Linux
kernel's wide array of primitives that cause code to
be invoked at a later time.

- o The pointer being compared against also came from
+ - The pointer being compared against also came from
rcu_dereference(). In this case, both pointers depend
on one rcu_dereference() or another, so you get proper
ordering either way.
@@ -159,13 +162,13 @@ o Be very careful about comparing pointers obtained from
of such an RCU usage bug is shown in the section titled
"EXAMPLE OF AMPLIFIED RCU-USAGE BUG".

- o All of the accesses following the comparison are stores,
+ - All of the accesses following the comparison are stores,
so that a control dependency preserves the needed ordering.
That said, it is easy to get control dependencies wrong.
Please see the "CONTROL DEPENDENCIES" section of
Documentation/memory-barriers.txt for more details.

- o The pointers are not equal -and- the compiler does
+ - The pointers are not equal -and- the compiler does
not have enough information to deduce the value of the
pointer. Note that the volatile cast in rcu_dereference()
will normally prevent the compiler from knowing too much.
@@ -175,7 +178,7 @@ o Be very careful about comparing pointers obtained from
comparison will provide exactly the information that the
compiler needs to deduce the value of the pointer.

-o Disable any value-speculation optimizations that your compiler
+- Disable any value-speculation optimizations that your compiler
might provide, especially if you are making use of feedback-based
optimizations that take data collected from prior runs. Such
value-speculation optimizations reorder operations by design.
@@ -188,11 +191,12 @@ o Disable any value-speculation optimizations that your compiler


EXAMPLE OF AMPLIFIED RCU-USAGE BUG
+----------------------------------

Because updaters can run concurrently with RCU readers, RCU readers can
see stale and/or inconsistent values. If RCU readers need fresh or
consistent values, which they sometimes do, they need to take proper
-precautions. To see this, consider the following code fragment:
+precautions. To see this, consider the following code fragment::

struct foo {
int a;
@@ -244,7 +248,7 @@ to some reordering from the compiler and CPUs is beside the point.

But suppose that the reader needs a consistent view?

-Then one approach is to use locking, for example, as follows:
+Then one approach is to use locking, for example, as follows::

struct foo {
int a;
@@ -299,6 +303,7 @@ As always, use the right tool for the job!


EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH
+-----------------------------------------

If a pointer obtained from rcu_dereference() compares not-equal to some
other pointer, the compiler normally has no clue what the value of the
@@ -308,7 +313,7 @@ guarantees that RCU depends on. And the volatile cast in rcu_dereference()
should prevent the compiler from guessing the value.

But without rcu_dereference(), the compiler knows more than you might
-expect. Consider the following code fragment:
+expect. Consider the following code fragment::

struct foo {
int a;
@@ -354,6 +359,7 @@ dereference the resulting pointer.


WHICH MEMBER OF THE rcu_dereference() FAMILY SHOULD YOU USE?
+------------------------------------------------------------

First, please avoid using rcu_dereference_raw() and also please avoid
using rcu_dereference_check() and rcu_dereference_protected() with a
@@ -370,7 +376,7 @@ member of the rcu_dereference() to use in various situations:

2. If the access might be within an RCU read-side critical section
on the one hand, or protected by (say) my_lock on the other,
- use rcu_dereference_check(), for example:
+ use rcu_dereference_check(), for example::

p1 = rcu_dereference_check(p->rcu_protected_pointer,
lockdep_is_held(&my_lock));
@@ -378,14 +384,14 @@ member of the rcu_dereference() to use in various situations:

3. If the access might be within an RCU read-side critical section
on the one hand, or protected by either my_lock or your_lock on
- the other, again use rcu_dereference_check(), for example:
+ the other, again use rcu_dereference_check(), for example::

p1 = rcu_dereference_check(p->rcu_protected_pointer,
lockdep_is_held(&my_lock) ||
lockdep_is_held(&your_lock));

4. If the access is on the update side, so that it is always protected
- by my_lock, use rcu_dereference_protected():
+ by my_lock, use rcu_dereference_protected()::

p1 = rcu_dereference_protected(p->rcu_protected_pointer,
lockdep_is_held(&my_lock));
@@ -410,18 +416,19 @@ member of the rcu_dereference() to use in various situations:


SPARSE CHECKING OF RCU-PROTECTED POINTERS
+-----------------------------------------

The sparse static-analysis tool checks for direct access to RCU-protected
pointers, which can result in "interesting" bugs due to compiler
optimizations involving invented loads and perhaps also load tearing.
-For example, suppose someone mistakenly does something like this:
+For example, suppose someone mistakenly does something like this::

p = q->rcu_protected_pointer;
do_something_with(p->a);
do_something_else_with(p->b);

If register pressure is high, the compiler might optimize "p" out
-of existence, transforming the code to something like this:
+of existence, transforming the code to something like this::

do_something_with(q->rcu_protected_pointer->a);
do_something_else_with(q->rcu_protected_pointer->b);
@@ -435,7 +442,7 @@ Load tearing could of course result in dereferencing a mashup of a pair
of pointers, which also might fatally disappoint your code.

These problems could have been avoided simply by making the code instead
-read as follows:
+read as follows::

p = rcu_dereference(q->rcu_protected_pointer);
do_something_with(p->a);
@@ -448,7 +455,7 @@ or as a formal parameter, with "__rcu", which tells sparse to complain if
this pointer is accessed directly. It will also cause sparse to complain
if a pointer not marked with "__rcu" is accessed using rcu_dereference()
and friends. For example, ->rcu_protected_pointer might be declared as
-follows:
+follows::

struct foo __rcu *rcu_protected_pointer;

--
2.9.5
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