parallel lookups machinery, part 2

We'll need to verify that there's neither a hashed nor in-lookup
dentry with desired parent/name before adding to in-lookup set.

One possible solution would be to hold the parent's ->d_lock through
both checks, but while the in-lookup set is relatively small at any
time, dcache is not.  And holding the parent's ->d_lock through
something like __d_lookup_rcu() would suck too badly.

So we leave the parent's ->d_lock alone, which means that we watch
out for the following scenario:
	* we verify that there's no hashed match
	* existing in-lookup match gets hashed by another process
	* we verify that there's no in-lookup matches and decide
that everything's fine.

Solution: per-directory kinda-sorta seqlock, bumped around the times
we hash something that used to be in-lookup or move (and hash)
something in place of in-lookup.  Then the above would turn into
	* read the counter
	* do dcache lookup
	* if no matches found, check for in-lookup matches
	* if there had been none of those either, check if the
counter has changed; repeat if it has.

The "kinda-sorta" part is due to the fact that we don't have much spare
space in inode.  There is a spare word (shared with i_bdev/i_cdev/i_pipe),
so the counter part is not a problem, but spinlock is a different story.

We could use the parent's ->d_lock, and it would be less painful in
terms of contention, for __d_add() it would be rather inconvenient to
grab; we could do that (using lock_parent()), but...

Fortunately, we can get serialization on the counter itself, and it
might be a good idea in general; we can use cmpxchg() in a loop to
get from even to odd and smp_store_release() from odd to even.

This commit adds the counter and updating logics; the readers will be
added in the next commit.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 32 insertions, 2 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 0f1d93866e69..10988f7e5a23 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2364,6 +2364,22 @@ void d_rehash(struct dentry * entry)
 }
 EXPORT_SYMBOL(d_rehash);
 
+static inline unsigned start_dir_add(struct inode *dir)
+{
+
+        for (;;) {
+                unsigned n = dir->i_dir_seq;
+                if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
+                        return n;
+                cpu_relax();
+        }
+}
+
+static inline void end_dir_add(struct inode *dir, unsigned n)
+{
+        smp_store_release(&dir->i_dir_seq, n + 2);
+}
+
 void __d_lookup_done(struct dentry *dentry)
 {
         dentry->d_flags &= ~DCACHE_PAR_LOOKUP;
@@ -2375,9 +2391,14 @@ EXPORT_SYMBOL(__d_lookup_done);
 
 static inline void __d_add(struct dentry *dentry, struct inode *inode)
 {
+        struct inode *dir = NULL;
+        unsigned n;
         spin_lock(&dentry->d_lock);
-        if (unlikely(d_in_lookup(dentry)))
+        if (unlikely(d_in_lookup(dentry))) {
+                dir = dentry->d_parent->d_inode;
+                n = start_dir_add(dir);
                 __d_lookup_done(dentry);
+        }
         if (inode) {
                 unsigned add_flags = d_flags_for_inode(inode);
                 hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
@@ -2387,6 +2408,8 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode)
                 __fsnotify_d_instantiate(dentry);
         }
         _d_rehash(dentry);
+        if (dir)
+                end_dir_add(dir, n);
         spin_unlock(&dentry->d_lock);
         if (inode)
                 spin_unlock(&inode->i_lock);
@@ -2616,6 +2639,8 @@ static void dentry_unlock_for_move(struct dentry *dentry, struct dentry *target)
 static void __d_move(struct dentry *dentry, struct dentry *target,
                      bool exchange)
 {
+        struct inode *dir = NULL;
+        unsigned n;
         if (!dentry->d_inode)
                 printk(KERN_WARNING "VFS: moving negative dcache entry\n");
 
@@ -2623,8 +2648,11 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
         BUG_ON(d_ancestor(target, dentry));
 
         dentry_lock_for_move(dentry, target);
-        if (unlikely(d_in_lookup(target)))
+        if (unlikely(d_in_lookup(target))) {
+                dir = target->d_parent->d_inode;
+                n = start_dir_add(dir);
                 __d_lookup_done(target);
+        }
 
         write_seqcount_begin(&dentry->d_seq);
         write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);
@@ -2674,6 +2702,8 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
         write_seqcount_end(&target->d_seq);
         write_seqcount_end(&dentry->d_seq);
 
+        if (dir)
+                end_dir_add(dir, n);
         dentry_unlock_for_move(dentry, target);
 }