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Subject[PATCH tip/core/urgent 3/3] rcu: Update docs for rcu_access_pointer and rcu_dereference_protected
Date
Update examples and lists of APIs to include these new primitives.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/RCU/NMI-RCU.txt | 39 ++++++++++++++++++++++-----------------
Documentation/RCU/checklist.txt | 7 ++++---
Documentation/RCU/lockdep.txt | 28 ++++++++++++++++++++++++++--
Documentation/RCU/whatisRCU.txt | 6 ++++++
4 files changed, 58 insertions(+), 22 deletions(-)
diff --git a/Documentation/RCU/NMI-RCU.txt b/Documentation/RCU/NMI-RCU.txt
index a6d32e6..a8536cb 100644
--- a/Documentation/RCU/NMI-RCU.txt
+++ b/Documentation/RCU/NMI-RCU.txt
@@ -34,7 +34,7 @@ NMI handler.
cpu = smp_processor_id();
++nmi_count(cpu);

- if (!rcu_dereference(nmi_callback)(regs, cpu))
+ if (!rcu_dereference_sched(nmi_callback)(regs, cpu))
default_do_nmi(regs);

nmi_exit();
@@ -47,12 +47,13 @@ function pointer. If this handler returns zero, do_nmi() invokes the
default_do_nmi() function to handle a machine-specific NMI. Finally,
preemption is restored.

-Strictly speaking, rcu_dereference() is not needed, since this code runs
-only on i386, which does not need rcu_dereference() anyway. However,
-it is a good documentation aid, particularly for anyone attempting to
-do something similar on Alpha.
+In theory, rcu_dereference_sched() is not needed, since this code runs
+only on i386, which in theory does not need rcu_dereference_sched()
+anyway. However, in practice it is a good documentation aid, particularly
+for anyone attempting to do something similar on Alpha or on systems
+with aggressive optimizing compilers.

-Quick Quiz: Why might the rcu_dereference() be necessary on Alpha,
+Quick Quiz: Why might the rcu_dereference_sched() be necessary on Alpha,
given that the code referenced by the pointer is read-only?


@@ -99,17 +100,21 @@ invoke irq_enter() and irq_exit() on NMI entry and exit, respectively.

Answer to Quick Quiz

- Why might the rcu_dereference() be necessary on Alpha, given
+ Why might the rcu_dereference_sched() be necessary on Alpha, given
that the code referenced by the pointer is read-only?

Answer: The caller to set_nmi_callback() might well have
- initialized some data that is to be used by the
- new NMI handler. In this case, the rcu_dereference()
- would be needed, because otherwise a CPU that received
- an NMI just after the new handler was set might see
- the pointer to the new NMI handler, but the old
- pre-initialized version of the handler's data.
-
- More important, the rcu_dereference() makes it clear
- to someone reading the code that the pointer is being
- protected by RCU.
+ initialized some data that is to be used by the new NMI
+ handler. In this case, the rcu_dereference_sched() would
+ be needed, because otherwise a CPU that received an NMI
+ just after the new handler was set might see the pointer
+ to the new NMI handler, but the old pre-initialized
+ version of the handler's data.
+
+ This same sad story can happen on other CPUs when using
+ a compiler with aggressive pointer-value speculation
+ optimizations.
+
+ More important, the rcu_dereference_sched() makes it
+ clear to someone reading the code that the pointer is
+ being protected by RCU-sched.
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index cbc180f..790d1a8 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -260,7 +260,8 @@ over a rather long period of time, but improvements are always welcome!
The reason that it is permissible to use RCU list-traversal
primitives when the update-side lock is held is that doing so
can be quite helpful in reducing code bloat when common code is
- shared between readers and updaters.
+ shared between readers and updaters. Additional primitives
+ are provided for this case, as discussed in lockdep.txt.

10. Conversely, if you are in an RCU read-side critical section,
and you don't hold the appropriate update-side lock, you -must-
@@ -344,8 +345,8 @@ over a rather long period of time, but improvements are always welcome!
requiring SRCU's read-side deadlock immunity or low read-side
realtime latency.

- Note that, rcu_assign_pointer() and rcu_dereference() relate to
- SRCU just as they do to other forms of RCU.
+ Note that, rcu_assign_pointer() relates to SRCU just as they do
+ to other forms of RCU.

15. The whole point of call_rcu(), synchronize_rcu(), and friends
is to wait until all pre-existing readers have finished before
diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
index fe24b58..d7a49b2 100644
--- a/Documentation/RCU/lockdep.txt
+++ b/Documentation/RCU/lockdep.txt
@@ -32,9 +32,20 @@ checking of rcu_dereference() primitives:
srcu_dereference(p, sp):
Check for SRCU read-side critical section.
rcu_dereference_check(p, c):
- Use explicit check expression "c".
+ Use explicit check expression "c". This is useful in
+ code that is invoked by both readers and updaters.
rcu_dereference_raw(p)
Don't check. (Use sparingly, if at all.)
+ rcu_dereference_protected(p, c):
+ Use explicit check expression "c", and omit all barriers
+ and compiler constraints. This is useful when the data
+ structure cannot change, for example, in code that is
+ invoked only by updaters.
+ rcu_access_pointer(p):
+ Return the value of the pointer and omit all barriers,
+ 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.

The rcu_dereference_check() check expression can be any boolean
expression, but would normally include one of the rcu_read_lock_held()
@@ -59,7 +70,20 @@ In case (1), the pointer is picked up in an RCU-safe manner for vanilla
RCU read-side critical sections, in case (2) the ->file_lock prevents
any change from taking place, and finally, in case (3) the current task
is the only task accessing the file_struct, again preventing any change
-from taking place.
+from taking place. If the above statement was invoked only from updater
+code, it could instead be written as follows:
+
+ file = rcu_dereference_protected(fdt->fd[fd],
+ lockdep_is_held(&files->file_lock) ||
+ atomic_read(&files->count) == 1);
+
+This would verify cases #2 and #3 above, and furthermore lockdep would
+complain if this was used in an RCU read-side critical section unless one
+of these two cases held. Because rcu_dereference_protected() omits all
+barriers and compiler constraints, it generates better code than do the
+other flavors of rcu_dereference(). On the other hand, it is illegal
+to use rcu_dereference_protected() if either the RCU-protected pointer
+or the RCU-protected data that it points to can change concurrently.

There are currently only "universal" versions of the rcu_assign_pointer()
and RCU list-/tree-traversal primitives, which do not (yet) check for
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 1dc00ee..cfaac34 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -840,6 +840,12 @@ SRCU: Initialization/cleanup
init_srcu_struct
cleanup_srcu_struct

+All: lockdep-checked RCU-protected pointer access
+
+ rcu_dereference_check
+ rcu_dereference_protected
+ rcu_access_pointer
+
See the comment headers in the source code (or the docbook generated
from them) for more information.

--
1.7.0


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