[PATCH v4 11/14] locks: add a new "lm_owner_key" lock operation

[Jeff Layton]

Currently, the hashing that the locking code uses to add these values
to the blocked_hash is simply calculated using fl_owner field. That's
valid in most cases except for server-side lockd, which validates the
owner of a lock based on fl_owner and fl_pid.

In the case where you have a small number of NFS clients doing a lot
of locking between different processes, you could end up with all
the blocked requests sitting in a very small number of hash buckets.

Add a new lm_owner_key operation to the lock_manager_operations that
will generate an unsigned long to use as the key in the hashtable.
That function is only implemented for server-side lockd, and simply
XORs the fl_owner and fl_pid.

Signed-off-by: Jeff Layton <jlayton at redhat.com>
Acked-by: J. Bruce Fields <bfields at fieldses.org>
---
 Documentation/filesystems/Locking |   16 +++++++++++-----
 fs/lockd/svclock.c                |   12 ++++++++++++
 fs/locks.c                        |   12 ++++++++++--
 include/linux/fs.h                |    1 +
 4 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 413685f..dfeb01b 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -351,6 +351,7 @@ fl_release_private:	maybe		no
 ----------------------- lock_manager_operations ---------------------------
 prototypes:
 	int (*lm_compare_owner)(struct file_lock *, struct file_lock *);
+	unsigned long (*lm_owner_key)(struct file_lock *);
 	void (*lm_notify)(struct file_lock *);  /* unblock callback */
 	int (*lm_grant)(struct file_lock *, struct file_lock *, int);
 	void (*lm_break)(struct file_lock *); /* break_lease callback */
@@ -360,16 +361,21 @@ locking rules:
 
 			inode->i_lock	file_lock_lock	may block
 lm_compare_owner:	yes[1]		maybe		no
+lm_owner_key		yes[1]		yes		no
 lm_notify:		yes		yes		no
 lm_grant:		no		no		no
 lm_break:		yes		no		no
 lm_change		yes		no		no
 
-[1]:	->lm_compare_owner is generally called with *an* inode->i_lock held. It
-may not be the i_lock of the inode for either file_lock being compared! This is
-the case with deadlock detection, since the code has to chase down the owners
-of locks that may be entirely unrelated to the one on which the lock is being
-acquired. When doing a search for deadlocks, the file_lock_lock is also held.
+[1]:	->lm_compare_owner and ->lm_owner_key are generally called with
+*an* inode->i_lock held. It may not be the i_lock of the inode
+associated with either file_lock argument! This is the case with deadlock
+detection, since the code has to chase down the owners of locks that may
+be entirely unrelated to the one on which the lock is being acquired.
+For deadlock detection however, the file_lock_lock is also held. The
+fact that these locks are held ensures that the file_locks do not
+disappear out from under you while doing the comparison or generating an
+owner key.
 
 --------------------------- buffer_head -----------------------------------
 prototypes:
diff --git a/fs/lockd/svclock.c b/fs/lockd/svclock.c
index a469098..067778b 100644
--- a/fs/lockd/svclock.c
+++ b/fs/lockd/svclock.c
@@ -744,8 +744,20 @@ static int nlmsvc_same_owner(struct file_lock *fl1, struct file_lock *fl2)
 	return fl1->fl_owner == fl2->fl_owner && fl1->fl_pid == fl2->fl_pid;
 }
 
+/*
+ * Since NLM uses two "keys" for tracking locks, we need to hash them down
+ * to one for the blocked_hash. Here, we're just xor'ing the host address
+ * with the pid in order to create a key value for picking a hash bucket.
+ */
+static unsigned long
+nlmsvc_owner_key(struct file_lock *fl)
+{
+	return (unsigned long)fl->fl_owner ^ (unsigned long)fl->fl_pid;
+}
+
 const struct lock_manager_operations nlmsvc_lock_operations = {
 	.lm_compare_owner = nlmsvc_same_owner,
+	.lm_owner_key = nlmsvc_owner_key,
 	.lm_notify = nlmsvc_notify_blocked,
 	.lm_grant = nlmsvc_grant_deferred,
 };
diff --git a/fs/locks.c b/fs/locks.c
index 71d847c..6242e0b 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -521,10 +521,18 @@ locks_delete_global_locks(struct file_lock *fl)
 	spin_unlock(&file_lock_lock);
 }
 
+static unsigned long
+posix_owner_key(struct file_lock *fl)
+{
+	if (fl->fl_lmops && fl->fl_lmops->lm_owner_key)
+		return fl->fl_lmops->lm_owner_key(fl);
+	return (unsigned long)fl->fl_owner;
+}
+
 static inline void
 locks_insert_global_blocked(struct file_lock *waiter)
 {
-	hash_add(blocked_hash, &waiter->fl_link, (unsigned long)waiter->fl_owner);
+	hash_add(blocked_hash, &waiter->fl_link, posix_owner_key(waiter));
 }
 
 static inline void
@@ -757,7 +765,7 @@ static struct file_lock *what_owner_is_waiting_for(struct file_lock *block_fl)
 {
 	struct file_lock *fl;
 
-	hash_for_each_possible(blocked_hash, fl, fl_link, (unsigned long)block_fl->fl_owner) {
+	hash_for_each_possible(blocked_hash, fl, fl_link, posix_owner_key(block_fl)) {
 		if (posix_same_owner(fl, block_fl))
 			return fl->fl_next;
 	}
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 85ef56a..e42e04f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -908,6 +908,7 @@ struct file_lock_operations {
 
 struct lock_manager_operations {
 	int (*lm_compare_owner)(struct file_lock *, struct file_lock *);
+	unsigned long (*lm_owner_key)(struct file_lock *);
 	void (*lm_notify)(struct file_lock *);	/* unblock callback */
 	int (*lm_grant)(struct file_lock *, struct file_lock *, int);
 	void (*lm_break)(struct file_lock *);
-- 
1.7.1