[PATCH] RCU documentation fixes (January 2006 update)
From: Paul E. McKenney
Date: Tue Jan 31 2006 - 18:36:05 EST
Hello!
Updates to in-tree RCU documentation based on comments over the past
few months.
Signed-off-by: <paulmck@xxxxxxxxxx>
---
RTFP.txt | 25 ++++++++++++++-----------
checklist.txt | 6 ++++++
listRCU.txt | 19 +++++++++++--------
rcu.txt | 5 +++++
rcuref.txt | 31 +++++++++++++++----------------
whatisRCU.txt | 31 +++++++++++++++++--------------
6 files changed, 68 insertions(+), 49 deletions(-)
diff -urpNa -X dontdiff linux-2.6.16-rc1/Documentation/RCU/checklist.txt linux-2.6.16-rc1-RCUdoc/Documentation/RCU/checklist.txt
--- linux-2.6.16-rc1/Documentation/RCU/checklist.txt 2006-01-25 20:37:59.000000000 -0800
+++ linux-2.6.16-rc1-RCUdoc/Documentation/RCU/checklist.txt 2006-01-25 20:38:26.000000000 -0800
@@ -177,3 +177,9 @@ over a rather long period of time, but i
If you want to wait for some of these other things, you might
instead need to use synchronize_irq() or synchronize_sched().
+
+12. Any lock acquired by an RCU callback must be acquired elsewhere
+ with irq disabled, e.g., via spin_lock_irqsave(). Failing to
+ disable irq on a given acquisition of that lock will result in
+ deadlock as soon as the RCU callback happens to interrupt that
+ acquisition's critical section.
diff -urpNa -X dontdiff linux-2.6.16-rc1/Documentation/RCU/listRCU.txt linux-2.6.16-rc1-RCUdoc/Documentation/RCU/listRCU.txt
--- linux-2.6.16-rc1/Documentation/RCU/listRCU.txt 2006-01-16 23:44:47.000000000 -0800
+++ linux-2.6.16-rc1-RCUdoc/Documentation/RCU/listRCU.txt 2006-01-25 20:32:15.000000000 -0800
@@ -232,7 +232,7 @@ entry does not exist. For this to be he
return holding the per-entry spinlock, as ipc_lock() does in fact do.
Quick Quiz: Why does the search function need to return holding the
-per-entry lock for this deleted-flag technique to be helpful?
+ per-entry lock for this deleted-flag technique to be helpful?
If the system-call audit module were to ever need to reject stale data,
one way to accomplish this would be to add a "deleted" flag and a "lock"
@@ -275,8 +275,8 @@ flag under the spinlock as follows:
{
struct audit_entry *e;
- /* Do not use the _rcu iterator here, since this is the only
- * deletion routine. */
+ /* Do not need to use the _rcu iterator here, since this
+ * is the only deletion routine. */
list_for_each_entry(e, list, list) {
if (!audit_compare_rule(rule, &e->rule)) {
spin_lock(&e->lock);
@@ -304,9 +304,12 @@ function to reject newly deleted data.
Answer to Quick Quiz
+ Why does the search function need to return holding the per-entry
+ lock for this deleted-flag technique to be helpful?
-If the search function drops the per-entry lock before returning, then
-the caller will be processing stale data in any case. If it is really
-OK to be processing stale data, then you don't need a "deleted" flag.
-If processing stale data really is a problem, then you need to hold the
-per-entry lock across all of the code that uses the value looked up.
+ If the search function drops the per-entry lock before returning,
+ then the caller will be processing stale data in any case. If it
+ is really OK to be processing stale data, then you don't need a
+ "deleted" flag. If processing stale data really is a problem,
+ then you need to hold the per-entry lock across all of the code
+ that uses the value that was returned.
diff -urpNa -X dontdiff linux-2.6.16-rc1/Documentation/RCU/rcuref.txt linux-2.6.16-rc1-RCUdoc/Documentation/RCU/rcuref.txt
--- linux-2.6.16-rc1/Documentation/RCU/rcuref.txt 2006-01-16 23:44:47.000000000 -0800
+++ linux-2.6.16-rc1-RCUdoc/Documentation/RCU/rcuref.txt 2006-01-25 20:13:11.000000000 -0800
@@ -1,7 +1,7 @@
-Refcounter design for elements of lists/arrays protected by RCU.
+Reference-count design for elements of lists/arrays protected by RCU.
-Refcounting on elements of lists which are protected by traditional
-reader/writer spinlocks or semaphores are straight forward as in:
+Reference counting on elements of lists which are protected by traditional
+reader/writer spinlocks or semaphores are straightforward:
1. 2.
add() search_and_reference()
@@ -28,12 +28,12 @@ release_referenced() delete()
...
}
-If this list/array is made lock free using rcu as in changing the
-write_lock in add() and delete() to spin_lock and changing read_lock
+If this list/array is made lock free using RCU as in changing the
+write_lock() in add() and delete() to spin_lock and changing read_lock
in search_and_reference to rcu_read_lock(), the atomic_get in
search_and_reference could potentially hold reference to an element which
-has already been deleted from the list/array. atomic_inc_not_zero takes
-care of this scenario. search_and_reference should look as;
+has already been deleted from the list/array. Use atomic_inc_not_zero()
+in this scenario as follows:
1. 2.
add() search_and_reference()
@@ -51,17 +51,16 @@ add() search_and_reference()
release_referenced() delete()
{ {
... write_lock(&list_lock);
- atomic_dec(&el->rc, relfunc) ...
- ... delete_element
-} write_unlock(&list_lock);
- ...
+ if (atomic_dec_and_test(&el->rc)) ...
+ call_rcu(&el->head, el_free); delete_element
+ ... write_unlock(&list_lock);
+} ...
if (atomic_dec_and_test(&el->rc))
call_rcu(&el->head, el_free);
...
}
-Sometimes, reference to the element need to be obtained in the
-update (write) stream. In such cases, atomic_inc_not_zero might be an
-overkill since the spinlock serialising list updates are held. atomic_inc
-is to be used in such cases.
-
+Sometimes, a reference to the element needs to be obtained in the
+update (write) stream. In such cases, atomic_inc_not_zero() might be
+overkill, since we hold the update-side spinlock. One might instead
+use atomic_inc() in such cases.
diff -urpNa -X dontdiff linux-2.6.16-rc1/Documentation/RCU/rcu.txt linux-2.6.16-rc1-RCUdoc/Documentation/RCU/rcu.txt
--- linux-2.6.16-rc1/Documentation/RCU/rcu.txt 2006-01-16 23:44:47.000000000 -0800
+++ linux-2.6.16-rc1-RCUdoc/Documentation/RCU/rcu.txt 2006-01-25 19:55:26.000000000 -0800
@@ -111,6 +111,11 @@ o What are all these files in this direc
You are reading it!
+ rcuref.txt
+
+ Describes how to combine use of reference counts
+ with RCU.
+
whatisRCU.txt
Overview of how the RCU implementation works. Along
diff -urpNa -X dontdiff linux-2.6.16-rc1/Documentation/RCU/RTFP.txt linux-2.6.16-rc1-RCUdoc/Documentation/RCU/RTFP.txt
--- linux-2.6.16-rc1/Documentation/RCU/RTFP.txt 2006-01-16 23:44:47.000000000 -0800
+++ linux-2.6.16-rc1-RCUdoc/Documentation/RCU/RTFP.txt 2006-01-25 19:44:02.000000000 -0800
@@ -90,16 +90,20 @@ at OLS. The resulting abundance of RCU
following year [McKenney02a], and use of RCU in dcache was first
described that same year [Linder02a].
-Also in 2002, Michael [Michael02b,Michael02a] presented techniques
-that defer the destruction of data structures to simplify non-blocking
-synchronization (wait-free synchronization, lock-free synchronization,
-and obstruction-free synchronization are all examples of non-blocking
-synchronization). In particular, this technique eliminates locking,
-reduces contention, reduces memory latency for readers, and parallelizes
-pipeline stalls and memory latency for writers. However, these
-techniques still impose significant read-side overhead in the form of
-memory barriers. Researchers at Sun worked along similar lines in the
-same timeframe [HerlihyLM02,HerlihyLMS03].
+Also in 2002, Michael [Michael02b,Michael02a] presented "hazard-pointer"
+techniques that defer the destruction of data structures to simplify
+non-blocking synchronization (wait-free synchronization, lock-free
+synchronization, and obstruction-free synchronization are all examples of
+non-blocking synchronization). In particular, this technique eliminates
+locking, reduces contention, reduces memory latency for readers, and
+parallelizes pipeline stalls and memory latency for writers. However,
+these techniques still impose significant read-side overhead in the
+form of memory barriers. Researchers at Sun worked along similar lines
+in the same timeframe [HerlihyLM02,HerlihyLMS03]. These techniques
+can be thought of as inside-out reference counts, where the count is
+represented by the number of hazard pointers referencing a given data
+structure (rather than the more conventional counter field within the
+data structure itself).
In 2003, the K42 group described how RCU could be used to create
hot-pluggable implementations of operating-system functions. Later that
@@ -113,7 +117,6 @@ number of operating-system kernels [Paul
describing how to make RCU safe for soft-realtime applications [Sarma04c],
and a paper describing SELinux performance with RCU [JamesMorris04b].
-
2005 has seen further adaptation of RCU to realtime use, permitting
preemption of RCU realtime critical sections [PaulMcKenney05a,
PaulMcKenney05b].
diff -urpNa -X dontdiff linux-2.6.16-rc1/Documentation/RCU/whatisRCU.txt linux-2.6.16-rc1-RCUdoc/Documentation/RCU/whatisRCU.txt
--- linux-2.6.16-rc1/Documentation/RCU/whatisRCU.txt 2006-01-25 16:51:42.000000000 -0800
+++ linux-2.6.16-rc1-RCUdoc/Documentation/RCU/whatisRCU.txt 2006-01-26 17:59:39.000000000 -0800
@@ -200,12 +200,11 @@ rcu_assign_pointer()
the new value, and also executes any memory-barrier instructions
required for a given CPU architecture.
- Perhaps just as important, it serves to document which pointers
- are protected by RCU. That said, rcu_assign_pointer() is most
- frequently used indirectly, via the _rcu list-manipulation
- primitives such as list_add_rcu(). The primitives
- rcu_assign_pointer(), list_add_rcu(), hlist_add_head(), and
- friends mark the point at which the structure is accessible
+ Perhaps just as important, it serves to document (1) which
+ pointers are protected by RCU and (2) the point at which a
+ given structure becomes accessible to other CPUs. That said,
+ rcu_assign_pointer() is most frequently used indirectly, via
+ the _rcu list-manipulation primitives such as list_add_rcu().
rcu_dereference()
@@ -260,9 +259,11 @@ rcu_dereference()
locking.
As with rcu_assign_pointer(), an important function of
- rcu_dereference() is to document which pointers are protected
- by RCU. And, again like rcu_assign_pointer(), rcu_dereference()
- is typically used indirectly, via the _rcu list-manipulation
+ rcu_dereference() is to document which pointers are protected by
+ RCU, in particular, flagging a pointer that is subject to changing
+ at any time, including immediately after the rcu_dereference().
+ And, again like rcu_assign_pointer(), rcu_dereference() is
+ typically used indirectly, via the _rcu list-manipulation
primitives, such as list_for_each_entry_rcu().
The following diagram shows how each API communicates among the
@@ -329,7 +330,7 @@ for specialized uses, but are relatively
3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
This section shows a simple use of the core RCU API to protect a
-global pointer to a dynamically allocated structure. More typical
+global pointer to a dynamically allocated structure. More-typical
uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
struct foo {
@@ -412,6 +413,8 @@ o Use synchronize_rcu() -after- removing
data item.
See checklist.txt for additional rules to follow when using RCU.
+And again, more-typical uses of RCU may be found in listRCU.txt,
+arrayRCU.txt, and NMI-RCU.txt.
4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
@@ -515,7 +518,7 @@ production-quality implementation, and s
for papers describing the Linux kernel RCU implementation. The OLS'01
and OLS'02 papers are a good introduction, and the dissertation provides
-more details on the current implementation.
+more details on the current implementation as of early 2004.
5A. "TOY" IMPLEMENTATION #1: LOCKING
@@ -770,7 +773,6 @@ RCU pointer/list traversal:
rcu_dereference
list_for_each_rcu (to be deprecated in favor of
list_for_each_entry_rcu)
- list_for_each_safe_rcu (deprecated, not used)
list_for_each_entry_rcu
list_for_each_continue_rcu (to be deprecated in favor of new
list_for_each_entry_continue_rcu)
@@ -809,7 +811,8 @@ Quick Quiz #1: Why is this argument naiv
Answer: Consider the following sequence of events:
1. CPU 0 acquires some unrelated lock, call it
- "problematic_lock".
+ "problematic_lock", disabling irq via
+ spin_lock_irqsave().
2. CPU 1 enters synchronize_rcu(), write-acquiring
rcu_gp_mutex.
@@ -896,7 +899,7 @@ Answer: Just as PREEMPT_RT permits pree
ACKNOWLEDGEMENTS
My thanks to the people who helped make this human-readable, including
-Jon Walpole, Josh Triplett, Serge Hallyn, and Suzanne Wood.
+Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
For more information, see http://www.rdrop.com/users/paulmck/RCU.
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