NFSv4: Avoid thundering herd issues with nfs_release_seqid

Store a pointer to the rpc_task in struct nfs_seqid so that we can wake up
only that request that is able to grab the lock after we've released it.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

2 files changed, 16 insertions, 6 deletions

diff --git a/fs/nfs/nfs4_fs.h b/fs/nfs/nfs4_fs.h
index c45c21a5470f..b133b50dec9a 100644
--- a/fs/nfs/nfs4_fs.h
+++ b/fs/nfs/nfs4_fs.h
@@ -71,6 +71,7 @@ struct nfs_seqid_counter {
 struct nfs_seqid {
         struct nfs_seqid_counter *sequence;
         struct list_head list;
+        struct rpc_task *task;
 };
 
 static inline void nfs_confirm_seqid(struct nfs_seqid_counter *seqid, int status)
diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
index f0e9881c2aa2..7d098604802c 100644
--- a/fs/nfs/nfs4state.c
+++ b/fs/nfs/nfs4state.c
@@ -916,20 +916,28 @@ struct nfs_seqid *nfs_alloc_seqid(struct nfs_seqid_counter *counter, gfp_t gfp_m
         if (new != NULL) {
                 new->sequence = counter;
                 INIT_LIST_HEAD(&new->list);
+                new->task = NULL;
         }
         return new;
 }
 
 void nfs_release_seqid(struct nfs_seqid *seqid)
 {
-        if (!list_empty(&seqid->list)) {
-                struct nfs_seqid_counter *sequence = seqid->sequence;
+        struct nfs_seqid_counter *sequence;
 
-                spin_lock(&sequence->lock);
-                list_del_init(&seqid->list);
-                spin_unlock(&sequence->lock);
-                rpc_wake_up(&sequence->wait);
+        if (list_empty(&seqid->list))
+                return;
+        sequence = seqid->sequence;
+        spin_lock(&sequence->lock);
+        list_del_init(&seqid->list);
+        if (!list_empty(&sequence->list)) {
+                struct nfs_seqid *next;
+
+                next = list_first_entry(&sequence->list,
+                                struct nfs_seqid, list);
+                rpc_wake_up_queued_task(&sequence->wait, next->task);
         }
+        spin_unlock(&sequence->lock);
 }
 
 void nfs_free_seqid(struct nfs_seqid *seqid)
@@ -1000,6 +1008,7 @@ int nfs_wait_on_sequence(struct nfs_seqid *seqid, struct rpc_task *task)
         int status = 0;
 
         spin_lock(&sequence->lock);
+        seqid->task = task;
         if (list_empty(&seqid->list))
                 list_add_tail(&seqid->list, &sequence->list);
         if (list_first_entry(&sequence->list, struct nfs_seqid, list) == seqid)