[PATCH] inotify: lock avoidance with parent watch status in dentry

Previous inotify work avoidance is good when inotify is completely unused,
but it breaks down if even a single watch is in place anywhere in the
system.  Robin Holt notices that udev is one such culprit - it slows down a
512-thread application on a 512 CPU system from 6 seconds to 22 minutes.

Solve this by adding a flag in the dentry that tells inotify whether or not
its parent inode has a watch on it.  Event queueing to parent will skip
taking locks if this flag is cleared.  Setting and clearing of this flag on
all child dentries versus event delivery: this is no in terms of race
cases, and that was shown to be equivalent to always performing the check.

The essential behaviour is that activity occuring _after_ a watch has been
added and _before_ it has been removed, will generate events.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Robert Love <rml@novell.com>
Cc: John McCutchan <ttb@tentacle.dhs.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/linux/inotify.h b/include/linux/inotify.h
index 267c88b5f742..09e00433c78e 100644
--- a/include/linux/inotify.h
+++ b/include/linux/inotify.h
@@ -71,6 +71,8 @@ struct inotify_event {
 
 #ifdef CONFIG_INOTIFY
 
+extern void inotify_d_instantiate(struct dentry *, struct inode *);
+extern void inotify_d_move(struct dentry *);
 extern void inotify_inode_queue_event(struct inode *, __u32, __u32,
                                       const char *);
 extern void inotify_dentry_parent_queue_event(struct dentry *, __u32, __u32,
@@ -81,6 +83,15 @@ extern u32 inotify_get_cookie(void);
 
 #else
 
+static inline void inotify_d_instantiate(struct dentry *dentry,
+                                        struct inode *inode)
+{
+}
+
+static inline void inotify_d_move(struct dentry *dentry)
+{
+}
+
 static inline void inotify_inode_queue_event(struct inode *inode,
                                              __u32 mask, __u32 cookie,
                                              const char *filename)