diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2011-03-16 08:10:07 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2011-03-16 08:10:07 -0700
commit016aa2ed1cc9cf704cf76d8df07751b6daa9750f (patch)
parent34d211a2d5df4984a35b18d8ccacbe1d10abb067 (diff)
parent241e6663b5151733294d1a230a3fd8a4d32e187f (diff)
Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip
* 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip: smp: Document transitivity for memory barriers. rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT. rcupdate: remove dead code rcu: add documentation saying which RCU flavor to choose rcutorture: Get rid of duplicate sched.h include rcu: call __rcu_read_unlock() in exit_rcu for tiny RCU
5 files changed, 95 insertions, 7 deletions
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index cfaac34c4557..6ef692667e2f 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -849,6 +849,37 @@ All: lockdep-checked RCU-protected pointer access
See the comment headers in the source code (or the docbook generated
from them) for more information.
+However, given that there are no fewer than four families of RCU APIs
+in the Linux kernel, how do you choose which one to use? The following
+list can be helpful:
+a. Will readers need to block? If so, you need SRCU.
+b. What about the -rt patchset? If readers would need to block
+ in an non-rt kernel, you need SRCU. If readers would block
+ in a -rt kernel, but not in a non-rt kernel, SRCU is not
+ necessary.
+c. Do you need to treat NMI handlers, hardirq handlers,
+ and code segments with preemption disabled (whether
+ via preempt_disable(), local_irq_save(), local_bh_disable(),
+ or some other mechanism) as if they were explicit RCU readers?
+ If so, you need RCU-sched.
+d. Do you need RCU grace periods to complete even in the face
+ of softirq monopolization of one or more of the CPUs? For
+ example, is your code subject to network-based denial-of-service
+ attacks? If so, you need RCU-bh.
+e. Is your workload too update-intensive for normal use of
+ RCU, but inappropriate for other synchronization mechanisms?
+ If so, consider SLAB_DESTROY_BY_RCU. But please be careful!
+f. Otherwise, use RCU.
+Of course, this all assumes that you have determined that RCU is in fact
+the right tool for your job.
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 631ad2f1b229..f0d3a8026a56 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -21,6 +21,7 @@ Contents:
- SMP barrier pairing.
- Examples of memory barrier sequences.
- Read memory barriers vs load speculation.
+ - Transitivity
(*) Explicit kernel barriers.
@@ -959,6 +960,63 @@ the speculation will be cancelled and the value reloaded:
retrieved : : +-------+
+Transitivity is a deeply intuitive notion about ordering that is not
+always provided by real computer systems. The following example
+demonstrates transitivity (also called "cumulativity"):
+ CPU 1 CPU 2 CPU 3
+ ======================= ======================= =======================
+ { X = 0, Y = 0 }
+ <general barrier> <general barrier>
+Suppose that CPU 2's load from X returns 1 and its load from Y returns 0.
+This indicates that CPU 2's load from X in some sense follows CPU 1's
+store to X and that CPU 2's load from Y in some sense preceded CPU 3's
+store to Y. The question is then "Can CPU 3's load from X return 0?"
+Because CPU 2's load from X in some sense came after CPU 1's store, it
+is natural to expect that CPU 3's load from X must therefore return 1.
+This expectation is an example of transitivity: if a load executing on
+CPU A follows a load from the same variable executing on CPU B, then
+CPU A's load must either return the same value that CPU B's load did,
+or must return some later value.
+In the Linux kernel, use of general memory barriers guarantees
+transitivity. Therefore, in the above example, if CPU 2's load from X
+returns 1 and its load from Y returns 0, then CPU 3's load from X must
+also return 1.
+However, transitivity is -not- guaranteed for read or write barriers.
+For example, suppose that CPU 2's general barrier in the above example
+is changed to a read barrier as shown below:
+ CPU 1 CPU 2 CPU 3
+ ======================= ======================= =======================
+ { X = 0, Y = 0 }
+ <read barrier> <general barrier>
+This substitution destroys transitivity: in this example, it is perfectly
+legal for CPU 2's load from X to return 1, its load from Y to return 0,
+and CPU 3's load from X to return 0.
+The key point is that although CPU 2's read barrier orders its pair
+of loads, it does not guarantee to order CPU 1's store. Therefore, if
+this example runs on a system where CPUs 1 and 2 share a store buffer
+or a level of cache, CPU 2 might have early access to CPU 1's writes.
+General barriers are therefore required to ensure that all CPUs agree
+on the combined order of CPU 1's and CPU 2's accesses.
+To reiterate, if your code requires transitivity, use general barriers
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index a23a57a976d1..f3240e987928 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -214,11 +214,12 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
* Ensure that queued callbacks are all executed.
* If we detect that we are nested in a RCU read-side critical
* section, we should simply fail, otherwise we would deadlock.
+ * Note that the machinery to reliably determine whether
+ * or not we are in an RCU read-side critical section
+ * exists only in the preemptible RCU implementations
+ * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
+ * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
- WARN_ON(1);
- return 0;
if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
irqs_disabled()) {
@@ -229,7 +230,6 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
debug_object_free(head, &rcuhead_debug_descr);
return 1;
return 0;
diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
index 015abaea962a..3cb8e362e883 100644
--- a/kernel/rcutiny_plugin.h
+++ b/kernel/rcutiny_plugin.h
@@ -852,7 +852,7 @@ void exit_rcu(void)
if (t->rcu_read_lock_nesting == 0)
t->rcu_read_lock_nesting = 1;
- rcu_read_unlock();
+ __rcu_read_unlock();
#else /* #ifdef CONFIG_TINY_PREEMPT_RCU */
diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 89613f97ff26..c224da41890c 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -47,7 +47,6 @@
#include <linux/srcu.h>
#include <linux/slab.h>
#include <asm/byteorder.h>
-#include <linux/sched.h>
MODULE_AUTHOR("Paul E. McKenney <paulmck@us.ibm.com> and "