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authorPaul E. McKenney <paulmck@us.ibm.com>2005-05-01 08:59:05 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-05-01 08:59:05 -0700
commita83f1fe27f7252a2b73b4f22066e92bf99bd595b (patch)
treeb6c4dd485fff268f86f396a85aad88cf29783dcd /Documentation/RCU/listRCU.txt
parentfbd568a3e61a7decb8a754ad952aaa5b5c82e9e5 (diff)
downloadlinux-linaro-stable-a83f1fe27f7252a2b73b4f22066e92bf99bd595b.tar.gz
[PATCH] Update RCU documentation
Update the RCU documentation to allow for the new synchronize_rcu() and synchronize_sched() primitives. Fix a few other nits as well. Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'Documentation/RCU/listRCU.txt')
-rw-r--r--Documentation/RCU/listRCU.txt13
1 files changed, 9 insertions, 4 deletions
diff --git a/Documentation/RCU/listRCU.txt b/Documentation/RCU/listRCU.txt
index bda6ead69bd0..f8a54fa0d8ab 100644
--- a/Documentation/RCU/listRCU.txt
+++ b/Documentation/RCU/listRCU.txt
@@ -32,6 +32,7 @@ implementation of audit_filter_task() might be as follows:
enum audit_state state;
read_lock(&auditsc_lock);
+ /* Note: audit_netlink_sem held by caller. */
list_for_each_entry(e, &audit_tsklist, list) {
if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
read_unlock(&auditsc_lock);
@@ -55,6 +56,7 @@ This means that RCU can be easily applied to the read side, as follows:
enum audit_state state;
rcu_read_lock();
+ /* Note: audit_netlink_sem held by caller. */
list_for_each_entry_rcu(e, &audit_tsklist, list) {
if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
rcu_read_unlock();
@@ -139,12 +141,15 @@ Normally, the write_lock() and write_unlock() would be replaced by
a spin_lock() and a spin_unlock(), but in this case, all callers hold
audit_netlink_sem, so no additional locking is required. The auditsc_lock
can therefore be eliminated, since use of RCU eliminates the need for
-writers to exclude readers.
+writers to exclude readers. Normally, the write_lock() calls would
+be converted into spin_lock() calls.
The list_del(), list_add(), and list_add_tail() primitives have been
replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu().
The _rcu() list-manipulation primitives add memory barriers that are
-needed on weakly ordered CPUs (most of them!).
+needed on weakly ordered CPUs (most of them!). The list_del_rcu()
+primitive omits the pointer poisoning debug-assist code that would
+otherwise cause concurrent readers to fail spectacularly.
So, when readers can tolerate stale data and when entries are either added
or deleted, without in-place modification, it is very easy to use RCU!
@@ -166,6 +171,7 @@ otherwise, the added fields would need to be filled in):
struct audit_newentry *ne;
write_lock(&auditsc_lock);
+ /* Note: audit_netlink_sem held by caller. */
list_for_each_entry(e, list, list) {
if (!audit_compare_rule(rule, &e->rule)) {
e->rule.action = newaction;
@@ -199,8 +205,7 @@ RCU ("read-copy update") its name. The RCU code is as follows:
audit_copy_rule(&ne->rule, &e->rule);
ne->rule.action = newaction;
ne->rule.file_count = newfield_count;
- list_add_rcu(ne, e);
- list_del(e);
+ list_replace_rcu(e, ne);
call_rcu(&e->rcu, audit_free_rule, e);
return 0;
}