[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, 77 insertions, 10 deletions

diff --git a/fs/inotify.c b/fs/inotify.c
index 0ee39ef591c6..a61e93e17853 100644
--- a/fs/inotify.c
+++ b/fs/inotify.c
@@ -38,7 +38,6 @@
 #include <asm/ioctls.h>
 
 static atomic_t inotify_cookie;
-static atomic_t inotify_watches;
 
 static kmem_cache_t *watch_cachep;
 static kmem_cache_t *event_cachep;
@@ -381,6 +380,48 @@ static int find_inode(const char __user *dirname, struct nameidata *nd,
 }
 
 /*
+ * inotify_inode_watched - returns nonzero if there are watches on this inode
+ * and zero otherwise.  We call this lockless, we do not care if we race.
+ */
+static inline int inotify_inode_watched(struct inode *inode)
+{
+        return !list_empty(&inode->inotify_watches);
+}
+
+/*
+ * Get child dentry flag into synch with parent inode.
+ * Flag should always be clear for negative dentrys.
+ */
+static void set_dentry_child_flags(struct inode *inode, int watched)
+{
+        struct dentry *alias;
+
+        spin_lock(&dcache_lock);
+        list_for_each_entry(alias, &inode->i_dentry, d_alias) {
+                struct dentry *child;
+
+                list_for_each_entry(child, &alias->d_subdirs, d_u.d_child) {
+                        if (!child->d_inode) {
+                                WARN_ON(child->d_flags & DCACHE_INOTIFY_PARENT_WATCHED);
+                                continue;
+                        }
+                        spin_lock(&child->d_lock);
+                        if (watched) {
+                                WARN_ON(child->d_flags &
+                                                DCACHE_INOTIFY_PARENT_WATCHED);
+                                child->d_flags |= DCACHE_INOTIFY_PARENT_WATCHED;
+                        } else {
+                                WARN_ON(!(child->d_flags &
+                                        DCACHE_INOTIFY_PARENT_WATCHED));
+                                child->d_flags&=~DCACHE_INOTIFY_PARENT_WATCHED;
+                        }
+                        spin_unlock(&child->d_lock);
+                }
+        }
+        spin_unlock(&dcache_lock);
+}
+
+/*
  * create_watch - creates a watch on the given device.
  *
  * Callers must hold dev->mutex.  Calls inotify_dev_get_wd() so may sleep.
@@ -426,7 +467,6 @@ static struct inotify_watch *create_watch(struct inotify_device *dev,
         get_inotify_watch(watch);
 
         atomic_inc(&dev->user->inotify_watches);
-        atomic_inc(&inotify_watches);
 
         return watch;
 }
@@ -458,8 +498,10 @@ static void remove_watch_no_event(struct inotify_watch *watch,
         list_del(&watch->i_list);
         list_del(&watch->d_list);
 
+        if (!inotify_inode_watched(watch->inode))
+                set_dentry_child_flags(watch->inode, 0);
+
         atomic_dec(&dev->user->inotify_watches);
-        atomic_dec(&inotify_watches);
         idr_remove(&dev->idr, watch->wd);
         put_inotify_watch(watch);
 }
@@ -481,16 +523,39 @@ static void remove_watch(struct inotify_watch *watch,struct inotify_device *dev)
         remove_watch_no_event(watch, dev);
 }
 
+/* Kernel API */
+
 /*
- * inotify_inode_watched - returns nonzero if there are watches on this inode
- * and zero otherwise.  We call this lockless, we do not care if we race.
+ * inotify_d_instantiate - instantiate dcache entry for inode
  */
-static inline int inotify_inode_watched(struct inode *inode)
+void inotify_d_instantiate(struct dentry *entry, struct inode *inode)
 {
-        return !list_empty(&inode->inotify_watches);
+        struct dentry *parent;
+
+        if (!inode)
+                return;
+
+        WARN_ON(entry->d_flags & DCACHE_INOTIFY_PARENT_WATCHED);
+        spin_lock(&entry->d_lock);
+        parent = entry->d_parent;
+        if (inotify_inode_watched(parent->d_inode))
+                entry->d_flags |= DCACHE_INOTIFY_PARENT_WATCHED;
+        spin_unlock(&entry->d_lock);
 }
 
-/* Kernel API */
+/*
+ * inotify_d_move - dcache entry has been moved
+ */
+void inotify_d_move(struct dentry *entry)
+{
+        struct dentry *parent;
+
+        parent = entry->d_parent;
+        if (inotify_inode_watched(parent->d_inode))
+                entry->d_flags |= DCACHE_INOTIFY_PARENT_WATCHED;
+        else
+                entry->d_flags &= ~DCACHE_INOTIFY_PARENT_WATCHED;
+}
 
 /**
  * inotify_inode_queue_event - queue an event to all watches on this inode
@@ -538,7 +603,7 @@ void inotify_dentry_parent_queue_event(struct dentry *dentry, u32 mask,
         struct dentry *parent;
         struct inode *inode;
 
-        if (!atomic_read (&inotify_watches))
+        if (!(dentry->d_flags & DCACHE_INOTIFY_PARENT_WATCHED))
                 return;
 
         spin_lock(&dentry->d_lock);
@@ -993,6 +1058,9 @@ asmlinkage long sys_inotify_add_watch(int fd, const char __user *path, u32 mask)
                 goto out;
         }
 
+        if (!inotify_inode_watched(inode))
+                set_dentry_child_flags(inode, 1);
+
         /* Add the watch to the device's and the inode's list */
         list_add(&watch->d_list, &dev->watches);
         list_add(&watch->i_list, &inode->inotify_watches);
@@ -1065,7 +1133,6 @@ static int __init inotify_setup(void)
         inotify_max_user_watches = 8192;
 
         atomic_set(&inotify_cookie, 0);
-        atomic_set(&inotify_watches, 0);
 
         watch_cachep = kmem_cache_create("inotify_watch_cache",
                                          sizeof(struct inotify_watch),