[lustre-devel] [PATCH v2] staging: lustre: obdclass: change object lookup to no wait mode

[James Simmons]

From: Lai Siyao <lai.siyao at intel.com>

Currently we set LU_OBJECT_HEARD_BANSHEE on object when we want
to remove object from cache, but this may lead to deadlock, because
when other process lookup such object, it needs to wait for this
object until release (done at last refcount put), while that process
maybe already hold an LDLM lock.

Now that current code can handle dying object correctly, we can just
return such object in lookup, thus the above deadlock can be avoided.

Signed-off-by: Lai Siyao <lai.siyao at intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-9049
Reviewed-on: https://review.whamcloud.com/26965
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev at intel.com>
Tested-by: Cliff White <cliff.white at intel.com>
Reviewed-by: Fan Yong <fan.yong at intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin at intel.com>
Signed-off-by: James Simmons <jsimmons at infradead.org>
---
Changelog:

v1) Initial patch that didn't apply to staging-testing branch
v2) Rebased after Neil's patches landed. Remove unlikely() test
    as requested by Dan Carpenter

 drivers/staging/lustre/lustre/obdclass/lu_object.c | 39 +++++-----------------
 1 file changed, 9 insertions(+), 30 deletions(-)

diff --git a/drivers/staging/lustre/lustre/obdclass/lu_object.c b/drivers/staging/lustre/lustre/obdclass/lu_object.c
index f14e350..e0abd4f 100644
--- a/drivers/staging/lustre/lustre/obdclass/lu_object.c
+++ b/drivers/staging/lustre/lustre/obdclass/lu_object.c
@@ -593,15 +593,10 @@ static struct lu_object *htable_lookup(struct lu_site *s,
 				       const struct lu_fid *f,
 				       __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;
-	wait_queue_entry_t waiter;
-
-retry:
-	ver = cfs_hash_bd_version_get(bd);
+	u64 ver = cfs_hash_bd_version_get(bd);
 
 	if (*version == ver)
 		return ERR_PTR(-ENOENT);
@@ -618,31 +613,13 @@ static struct lu_object *htable_lookup(struct lu_site *s,
 	}
 
 	h = container_of(hnode, struct lu_object_header, loh_hash);
-	if (likely(!lu_object_is_dying(h))) {
-		cfs_hash_get(s->ls_obj_hash, hnode);
-		lprocfs_counter_incr(s->ls_stats, LU_SS_CACHE_HIT);
-		if (!list_empty(&h->loh_lru)) {
-			list_del_init(&h->loh_lru);
-			percpu_counter_dec(&s->ls_lru_len_counter);
-		}
-		return lu_object_top(h);
+	cfs_hash_get(s->ls_obj_hash, hnode);
+	lprocfs_counter_incr(s->ls_stats, LU_SS_CACHE_HIT);
+	if (!list_empty(&h->loh_lru)) {
+		list_del_init(&h->loh_lru);
+		percpu_counter_dec(&s->ls_lru_len_counter);
 	}
-
-	/*
-	 * Lookup found an object being destroyed this object cannot be
-	 * returned (to assure that references to dying objects are eventually
-	 * drained), and moreover, lookup has to wait until object is freed.
-	 */
-
-	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);
-	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;
+	return lu_object_top(h);
 }
 
 /**
@@ -683,6 +660,8 @@ static void lu_object_limit(const struct lu_env *env, struct lu_device *dev)
 }
 
 /**
+ * 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.
-- 
1.8.3.1