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Message-ID: <20110316065835.GA29204@elte.hu>
Date: Wed, 16 Mar 2011 07:58:35 +0100
From: Ingo Molnar <mingo@...e.hu>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org,
"Paul E. McKenney" <paulmck@...ibm.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] RCU changes for v2.6.39
* Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Tue, Mar 15, 2011 at 12:42 PM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > Please pull the latest core-locking-for-linus git tree from:
> >
> > git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git core-locking-for-linus
>
> Something screwed up? This was the core-locking pull request, not the
> RCU pull. And there's no shortlog or diff..
Oops, indeed. The right one is:
Please pull the latest core-rcu-for-linus git tree from:
git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git core-rcu-for-linus
Thanks,
Ingo
------------------>
Amerigo Wang (1):
rcupdate: remove dead code
Jesper Juhl (1):
rcutorture: Get rid of duplicate sched.h include
Lai Jiangshan (1):
rcu: call __rcu_read_unlock() in exit_rcu for tiny RCU
Paul E. McKenney (3):
rcu: add documentation saying which RCU flavor to choose
rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.
smp: Document transitivity for memory barriers.
Documentation/RCU/whatisRCU.txt | 31 +++++++++++++++++++
Documentation/memory-barriers.txt | 58 +++++++++++++++++++++++++++++++++++++
kernel/rcupdate.c | 10 +++---
kernel/rcutiny_plugin.h | 2 +-
kernel/rcutorture.c | 1 -
5 files changed, 95 insertions(+), 7 deletions(-)
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index cfaac34..6ef6926 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.
+
8. ANSWERS TO QUICK QUIZZES
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 631ad2f..f0d3a80 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
+------------
+
+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 }
+ STORE X=1 LOAD X STORE Y=1
+ <general barrier> <general barrier>
+ LOAD Y LOAD X
+
+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 }
+ STORE X=1 LOAD X STORE Y=1
+ <read barrier> <general barrier>
+ LOAD Y LOAD X
+
+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
+throughout.
+
+
========================
EXPLICIT KERNEL BARRIERS
========================
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index a23a57a..f3240e9 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.
*/
-#ifndef CONFIG_PREEMPT
- WARN_ON(1);
- return 0;
-#else
if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
irqs_disabled()) {
WARN_ON(1);
@@ -229,7 +230,6 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
rcu_barrier_bh();
debug_object_free(head, &rcuhead_debug_descr);
return 1;
-#endif
default:
return 0;
}
diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
index 015abae..3cb8e36 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)
return;
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 89613f9..c224da4 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_LICENSE("GPL");
MODULE_AUTHOR("Paul E. McKenney <paulmck@...ibm.com> and "
--
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