pnfs: change layout state seqlock to a spinlock

This prepares for future changes, where the layout state needs
to change atomically with several other variables.  In particular,
it will need to know if lo->segs is empty, as we test that instead
of manipulating the NFS_LAYOUT_STATEID_SET bit.  Moreover, the
layoutstateid is not really a read-mostly structure, as it is
written almost as often as it is read.

The behavior of pnfs_get_layout_stateid is also slightly changed, so that
it no longer changes the stateid.  Its name is changed to +pnfs_choose_layoutget_stateid.

Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 23 insertions, 56 deletions

diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
index 6736f9e4f2e1..08313f536b45 100644
--- a/fs/nfs/pnfs.c
+++ b/fs/nfs/pnfs.c
@@ -258,9 +258,6 @@ pnfs_clear_lseg_list(struct pnfs_layout_hdr *lo, struct list_head *tmp_list)
         /* List does not take a reference, so no need for put here */
         list_del_init(&lo->plh_layouts);
         spin_unlock(&clp->cl_lock);
-        write_seqlock(&lo->plh_seqlock);
-        clear_bit(NFS_LAYOUT_STATEID_SET, &lo->plh_flags);
-        write_sequnlock(&lo->plh_seqlock);
 
         dprintk("%s:Return\n", __func__);
 }
@@ -319,69 +316,40 @@ pnfs_destroy_all_layouts(struct nfs_client *clp)
         }
 }
 
-/* update lo->plh_stateid with new if is more recent
- *
- * lo->plh_stateid could be the open stateid, in which case we just use what given.
- */
+/* update lo->plh_stateid with new if is more recent */
 static void
 pnfs_set_layout_stateid(struct pnfs_layout_hdr *lo,
                         const nfs4_stateid *new)
 {
-        nfs4_stateid *old = &lo->plh_stateid;
-        bool overwrite = false;
-
-        write_seqlock(&lo->plh_seqlock);
-        if (!test_bit(NFS_LAYOUT_STATEID_SET, &lo->plh_flags) ||
-            memcmp(old->stateid.other, new->stateid.other, sizeof(new->stateid.other)))
-                overwrite = true;
-        else {
-                u32 oldseq, newseq;
-
-                oldseq = be32_to_cpu(old->stateid.seqid);
-                newseq = be32_to_cpu(new->stateid.seqid);
-                if ((int)(newseq - oldseq) > 0)
-                        overwrite = true;
-        }
-        if (overwrite)
-                memcpy(&old->stateid, &new->stateid, sizeof(new->stateid));
-        write_sequnlock(&lo->plh_seqlock);
-}
-
-static void
-pnfs_layout_from_open_stateid(struct pnfs_layout_hdr *lo,
-                              struct nfs4_state *state)
-{
-        int seq;
+        u32 oldseq, newseq;
 
-        dprintk("--> %s\n", __func__);
-        write_seqlock(&lo->plh_seqlock);
-        do {
-                seq = read_seqbegin(&state->seqlock);
-                memcpy(lo->plh_stateid.data, state->stateid.data,
-                       sizeof(state->stateid.data));
-        } while (read_seqretry(&state->seqlock, seq));
-        set_bit(NFS_LAYOUT_STATEID_SET, &lo->plh_flags);
-        write_sequnlock(&lo->plh_seqlock);
-        dprintk("<-- %s\n", __func__);
+        oldseq = be32_to_cpu(lo->plh_stateid.stateid.seqid);
+        newseq = be32_to_cpu(new->stateid.seqid);
+        if ((int)(newseq - oldseq) > 0)
+                memcpy(&lo->plh_stateid, &new->stateid, sizeof(new->stateid));
 }
 
-void
-pnfs_get_layout_stateid(nfs4_stateid *dst, struct pnfs_layout_hdr *lo,
-                        struct nfs4_state *open_state)
+int
+pnfs_choose_layoutget_stateid(nfs4_stateid *dst, struct pnfs_layout_hdr *lo,
+                              struct nfs4_state *open_state)
 {
-        int seq;
+        int status = 0;
 
         dprintk("--> %s\n", __func__);
-        do {
-                seq = read_seqbegin(&lo->plh_seqlock);
-                if (!test_bit(NFS_LAYOUT_STATEID_SET, &lo->plh_flags)) {
-                        /* This will trigger retry of the read */
-                        pnfs_layout_from_open_stateid(lo, open_state);
-                } else
-                        memcpy(dst->data, lo->plh_stateid.data,
-                               sizeof(lo->plh_stateid.data));
-        } while (read_seqretry(&lo->plh_seqlock, seq));
+        spin_lock(&lo->plh_inode->i_lock);
+        if (list_empty(&lo->plh_segs)) {
+                int seq;
+
+                do {
+                        seq = read_seqbegin(&open_state->seqlock);
+                        memcpy(dst->data, open_state->stateid.data,
+                               sizeof(open_state->stateid.data));
+                } while (read_seqretry(&open_state->seqlock, seq));
+        } else
+                memcpy(dst->data, lo->plh_stateid.data, sizeof(lo->plh_stateid.data));
+        spin_unlock(&lo->plh_inode->i_lock);
         dprintk("<-- %s\n", __func__);
+        return status;
 }
 
 /*
@@ -496,7 +464,6 @@ alloc_init_layout_hdr(struct inode *ino)
         lo->plh_refcount = 1;
         INIT_LIST_HEAD(&lo->plh_layouts);
         INIT_LIST_HEAD(&lo->plh_segs);
-        seqlock_init(&lo->plh_seqlock);
         lo->plh_inode = ino;
         return lo;
 }