[lustre-devel] [PATCH 04/10] staging: lustre: lu_object: move retry logic inside htable_lookup

[NeilBrown]

The current retry logic, to wait when a 'dying' object is found,
spans multiple functions.  The process is attached to a waitqueue
and set TASK_UNINTERRUPTIBLE in htable_lookup, and this status
is passed back through lu_object_find_try() to lu_object_find_at()
where schedule() is called and the process is removed from the queue.

This can be simplified by moving all the logic (including
hashtable locking) inside htable_lookup(), which now never returns
EAGAIN.

Note that htable_lookup() is called with the hash bucket lock
held, and will drop and retake it if it needs to schedule.

I made this a 'goto' loop rather than a 'while(1)' loop as the
diff is easier to read.

Signed-off-by: NeilBrown <neilb at suse.com>
---
 drivers/staging/lustre/lustre/obdclass/lu_object.c |   70 ++++++++------------
 1 file changed, 27 insertions(+), 43 deletions(-)

diff --git a/drivers/staging/lustre/lustre/obdclass/lu_object.c b/drivers/staging/lustre/lustre/obdclass/lu_object.c
index bf5b309edeae..0c9419a6d5d6 100644
--- a/drivers/staging/lustre/lustre/obdclass/lu_object.c
+++ b/drivers/staging/lustre/lustre/obdclass/lu_object.c
@@ -584,16 +584,24 @@ void lu_object_print(const struct lu_env *env, void *cookie,
 }
 EXPORT_SYMBOL(lu_object_print);
 
+/*
+ * NOTE: htable_lookup() is called with the relevant
+ * hash bucket locked, but might drop and re-acquire the lock.
+ */
 static struct lu_object *htable_lookup(struct lu_site *s,
 				       struct cfs_hash_bd *bd,
 				       const struct lu_fid *f,
-				       wait_queue_entry_t *waiter,
 				       __u64 *version)
 {
+	struct cfs_hash		*hs = s->ls_obj_hash;
 	struct lu_site_bkt_data *bkt;
 	struct lu_object_header *h;
 	struct hlist_node	*hnode;
-	__u64  ver = cfs_hash_bd_version_get(bd);
+	__u64 ver;
+	wait_queue_entry_t waiter;
+
+retry:
+	ver = cfs_hash_bd_version_get(bd);
 
 	if (*version == ver)
 		return ERR_PTR(-ENOENT);
@@ -626,11 +634,15 @@ static struct lu_object *htable_lookup(struct lu_site *s,
 	 * drained), and moreover, lookup has to wait until object is freed.
 	 */
 
-	init_waitqueue_entry(waiter, current);
-	add_wait_queue(&bkt->lsb_marche_funebre, waiter);
+	init_waitqueue_entry(&waiter, current);
+	add_wait_queue(&bkt->lsb_marche_funebre, &waiter);
 	set_current_state(TASK_UNINTERRUPTIBLE);
 	lprocfs_counter_incr(s->ls_stats, LU_SS_CACHE_DEATH_RACE);
-	return ERR_PTR(-EAGAIN);
+	cfs_hash_bd_unlock(hs, bd, 1);
+	schedule();
+	remove_wait_queue(&bkt->lsb_marche_funebre, &waiter);
+	cfs_hash_bd_lock(hs, bd, 1);
+	goto retry;
 }
 
 /**
@@ -694,13 +706,14 @@ static struct lu_object *lu_object_new(const struct lu_env *env,
 }
 
 /**
- * Core logic of lu_object_find*() functions.
+ * Much like lu_object_find(), but top level device of object is specifically
+ * \a dev rather than top level device of the site. This interface allows
+ * objects of different "stacking" to be created within the same site.
  */
-static struct lu_object *lu_object_find_try(const struct lu_env *env,
-					    struct lu_device *dev,
-					    const struct lu_fid *f,
-					    const struct lu_object_conf *conf,
-					    wait_queue_entry_t *waiter)
+struct lu_object *lu_object_find_at(const struct lu_env *env,
+				    struct lu_device *dev,
+				    const struct lu_fid *f,
+				    const struct lu_object_conf *conf)
 {
 	struct lu_object      *o;
 	struct lu_object      *shadow;
@@ -726,8 +739,6 @@ static struct lu_object *lu_object_find_try(const struct lu_env *env,
 	 * It is unnecessary to perform lookup-alloc-lookup-insert, instead,
 	 * just alloc and insert directly.
 	 *
-	 * If dying object is found during index search, add @waiter to the
-	 * site wait-queue and return ERR_PTR(-EAGAIN).
 	 */
 	if (conf && conf->loc_flags & LOC_F_NEW)
 		return lu_object_new(env, dev, f, conf);
@@ -735,8 +746,9 @@ static struct lu_object *lu_object_find_try(const struct lu_env *env,
 	s  = dev->ld_site;
 	hs = s->ls_obj_hash;
 	cfs_hash_bd_get_and_lock(hs, (void *)f, &bd, 1);
-	o = htable_lookup(s, &bd, f, waiter, &version);
+	o = htable_lookup(s, &bd, f, &version);
 	cfs_hash_bd_unlock(hs, &bd, 1);
+
 	if (!IS_ERR(o) || PTR_ERR(o) != -ENOENT)
 		return o;
 
@@ -752,7 +764,7 @@ static struct lu_object *lu_object_find_try(const struct lu_env *env,
 
 	cfs_hash_bd_lock(hs, &bd, 1);
 
-	shadow = htable_lookup(s, &bd, f, waiter, &version);
+	shadow = htable_lookup(s, &bd, f, &version);
 	if (likely(PTR_ERR(shadow) == -ENOENT)) {
 		cfs_hash_bd_add_locked(hs, &bd, &o->lo_header->loh_hash);
 		cfs_hash_bd_unlock(hs, &bd, 1);
@@ -767,34 +779,6 @@ static struct lu_object *lu_object_find_try(const struct lu_env *env,
 	lu_object_free(env, o);
 	return shadow;
 }
-
-/**
- * Much like lu_object_find(), but top level device of object is specifically
- * \a dev rather than top level device of the site. This interface allows
- * objects of different "stacking" to be created within the same site.
- */
-struct lu_object *lu_object_find_at(const struct lu_env *env,
-				    struct lu_device *dev,
-				    const struct lu_fid *f,
-				    const struct lu_object_conf *conf)
-{
-	wait_queue_head_t	*wq;
-	struct lu_object	*obj;
-	wait_queue_entry_t	   wait;
-
-	while (1) {
-		obj = lu_object_find_try(env, dev, f, conf, &wait);
-		if (obj != ERR_PTR(-EAGAIN))
-			return obj;
-		/*
-		 * lu_object_find_try() already added waiter into the
-		 * wait queue.
-		 */
-		schedule();
-		wq = lu_site_wq_from_fid(dev->ld_site, (void *)f);
-		remove_wait_queue(wq, &wait);
-	}
-}
 EXPORT_SYMBOL(lu_object_find_at);
 
 /**