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Message-Id: <1363640108-15508-2-git-send-email-paulmck@linux.vnet.ibm.com>
Date: Mon, 18 Mar 2013 13:55:08 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
fweisbec@...il.com, sbw@....edu,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 2/2] rcu: Documentation update
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
This commit applies a few updates based on a quick review of the RCU
documentations.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
Documentation/RCU/checklist.txt | 26 ++++++++++++++++----------
Documentation/RCU/lockdep.txt | 5 +++++
Documentation/RCU/rcubarrier.txt | 15 ++++++++++++++-
3 files changed, 35 insertions(+), 11 deletions(-)
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 31ef8fe..79e789b8 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -217,9 +217,14 @@ over a rather long period of time, but improvements are always welcome!
whether the increased speed is worth it.
8. Although synchronize_rcu() is slower than is call_rcu(), it
- usually results in simpler code. So, unless update performance
- is critically important or the updaters cannot block,
- synchronize_rcu() should be used in preference to call_rcu().
+ usually results in simpler code. So, unless update performance is
+ critically important, the updaters cannot block, or the latency of
+ synchronize_rcu() is visible from userspace, synchronize_rcu()
+ should be used in preference to call_rcu(). Furthermore,
+ kfree_rcu() usually results in even simpler code than does
+ synchronize_rcu() without synchronize_rcu()'s multi-millisecond
+ latency. So please take advantage of kfree_rcu()'s "fire and
+ forget" memory-freeing capabilities where it applies.
An especially important property of the synchronize_rcu()
primitive is that it automatically self-limits: if grace periods
@@ -268,7 +273,8 @@ over a rather long period of time, but improvements are always welcome!
e. Periodically invoke synchronize_rcu(), permitting a limited
number of updates per grace period.
- The same cautions apply to call_rcu_bh() and call_rcu_sched().
+ The same cautions apply to call_rcu_bh(), call_rcu_sched(),
+ call_srcu(), and kfree_rcu().
9. All RCU list-traversal primitives, which include
rcu_dereference(), list_for_each_entry_rcu(), and
@@ -296,9 +302,9 @@ over a rather long period of time, but improvements are always welcome!
all currently executing rcu_read_lock()-protected RCU read-side
critical sections complete. It does -not- necessarily guarantee
that all currently running interrupts, NMIs, preempt_disable()
- code, or idle loops will complete. Therefore, if you do not have
- rcu_read_lock()-protected read-side critical sections, do -not-
- use synchronize_rcu().
+ code, or idle loops will complete. Therefore, if your
+ read-side critical sections are protected by something other
+ than rcu_read_lock(), do -not- use synchronize_rcu().
Similarly, disabling preemption is not an acceptable substitute
for rcu_read_lock(). Code that attempts to use preemption
@@ -401,9 +407,9 @@ over a rather long period of time, but improvements are always welcome!
read-side critical sections. It is the responsibility of the
RCU update-side primitives to deal with this.
-17. Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and
- the __rcu sparse checks to validate your RCU code. These
- can help find problems as follows:
+17. Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
+ __rcu sparse checks (enabled by CONFIG_SPARSE_RCU_POINTER) to
+ validate your RCU code. These can help find problems as follows:
CONFIG_PROVE_RCU: check that accesses to RCU-protected data
structures are carried out under the proper RCU
diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
index a102d4b..cd83d23 100644
--- a/Documentation/RCU/lockdep.txt
+++ b/Documentation/RCU/lockdep.txt
@@ -64,6 +64,11 @@ 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/rcubarrier.txt b/Documentation/RCU/rcubarrier.txt
index 38428c1..2e319d1 100644
--- a/Documentation/RCU/rcubarrier.txt
+++ b/Documentation/RCU/rcubarrier.txt
@@ -79,7 +79,20 @@ complete. Pseudo-code using rcu_barrier() is as follows:
2. Execute rcu_barrier().
3. Allow the module to be unloaded.
-The rcutorture module makes use of rcu_barrier in its exit function
+There are also rcu_barrier_bh(), rcu_barrier_sched(), and srcu_barrier()
+functions for the other flavors of RCU, and you of course must match
+the flavor of rcu_barrier() with that of call_rcu(). If your module
+uses multiple flavors of call_rcu(), then it must also use multiple
+flavors of rcu_barrier() when unloading that module. For example, if
+it uses call_rcu_bh(), call_srcu() on srcu_struct_1, and call_srcu() on
+srcu_struct_2(), then the following three lines of code will be required
+when unloading:
+
+ 1 rcu_barrier_bh();
+ 2 srcu_barrier(&srcu_struct_1);
+ 3 srcu_barrier(&srcu_struct_2);
+
+The rcutorture module makes use of rcu_barrier() in its exit function
as follows:
1 static void
--
1.8.1.5
--
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