Btrfs: switch the btrfs tree locks to reader/writer

The btrfs metadata btree is the source of significant
lock contention, especially in the root node.   This
commit changes our locking to use a reader/writer
lock.

The lock is built on top of rw spinlocks, and it
extends the lock tracking to remember if we have a
read lock or a write lock when we go to blocking.  Atomics
count the number of blocking readers or writers at any
given time.

It removes all of the adaptive spinning from the old code
and uses only the spinning/blocking hints inside of btrfs
to decide when it should continue spinning.

In read heavy workloads this is dramatically faster.  In write
heavy workloads we're still faster because of less contention
on the root node lock.

We suffer slightly in dbench because we schedule more often
during write locks, but all other benchmarks so far are improved.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 209 insertions, 59 deletions

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index d2431284b913..0ad48e782d37 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -54,8 +54,13 @@ noinline void btrfs_set_path_blocking(struct btrfs_path *p)
 {
         int i;
         for (i = 0; i < BTRFS_MAX_LEVEL; i++) {
-                if (p->nodes[i] && p->locks[i])
-                        btrfs_set_lock_blocking(p->nodes[i]);
+                if (!p->nodes[i] || !p->locks[i])
+                        continue;
+                btrfs_set_lock_blocking_rw(p->nodes[i], p->locks[i]);
+                if (p->locks[i] == BTRFS_READ_LOCK)
+                        p->locks[i] = BTRFS_READ_LOCK_BLOCKING;
+                else if (p->locks[i] == BTRFS_WRITE_LOCK)
+                        p->locks[i] = BTRFS_WRITE_LOCK_BLOCKING;
         }
 }
 
@@ -68,7 +73,7 @@ noinline void btrfs_set_path_blocking(struct btrfs_path *p)
  * for held
  */
 noinline void btrfs_clear_path_blocking(struct btrfs_path *p,
-                                        struct extent_buffer *held)
+                                        struct extent_buffer *held, int held_rw)
 {
         int i;
 
@@ -79,19 +84,29 @@ noinline void btrfs_clear_path_blocking(struct btrfs_path *p,
          * really sure by forcing the path to blocking before we clear
          * the path blocking.
          */
-        if (held)
-                btrfs_set_lock_blocking(held);
+        if (held) {
+                btrfs_set_lock_blocking_rw(held, held_rw);
+                if (held_rw == BTRFS_WRITE_LOCK)
+                        held_rw = BTRFS_WRITE_LOCK_BLOCKING;
+                else if (held_rw == BTRFS_READ_LOCK)
+                        held_rw = BTRFS_READ_LOCK_BLOCKING;
+        }
         btrfs_set_path_blocking(p);
 #endif
 
         for (i = BTRFS_MAX_LEVEL - 1; i >= 0; i--) {
-                if (p->nodes[i] && p->locks[i])
-                        btrfs_clear_lock_blocking(p->nodes[i]);
+                if (p->nodes[i] && p->locks[i]) {
+                        btrfs_clear_lock_blocking_rw(p->nodes[i], p->locks[i]);
+                        if (p->locks[i] == BTRFS_WRITE_LOCK_BLOCKING)
+                                p->locks[i] = BTRFS_WRITE_LOCK;
+                        else if (p->locks[i] == BTRFS_READ_LOCK_BLOCKING)
+                                p->locks[i] = BTRFS_READ_LOCK;
+                }
         }
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
         if (held)
-                btrfs_clear_lock_blocking(held);
+                btrfs_clear_lock_blocking_rw(held, held_rw);
 #endif
 }
 
@@ -119,7 +134,7 @@ noinline void btrfs_release_path(struct btrfs_path *p)
                 if (!p->nodes[i])
                         continue;
                 if (p->locks[i]) {
-                        btrfs_tree_unlock(p->nodes[i]);
+                        btrfs_tree_unlock_rw(p->nodes[i], p->locks[i]);
                         p->locks[i] = 0;
                 }
                 free_extent_buffer(p->nodes[i]);
@@ -167,6 +182,25 @@ struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
         return eb;
 }
 
+/* loop around taking references on and locking the root node of the
+ * tree until you end up with a lock on the root.  A locked buffer
+ * is returned, with a reference held.
+ */
+struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
+{
+        struct extent_buffer *eb;
+
+        while (1) {
+                eb = btrfs_root_node(root);
+                btrfs_tree_read_lock(eb);
+                if (eb == root->node)
+                        break;
+                btrfs_tree_read_unlock(eb);
+                free_extent_buffer(eb);
+        }
+        return eb;
+}
+
 /* cowonly root (everything not a reference counted cow subvolume), just get
  * put onto a simple dirty list.  transaction.c walks this to make sure they
  * get properly updated on disk.
@@ -862,7 +896,8 @@ static noinline int balance_level(struct btrfs_trans_handle *trans,
 
         mid = path->nodes[level];
 
-        WARN_ON(!path->locks[level]);
+        WARN_ON(path->locks[level] != BTRFS_WRITE_LOCK &&
+                path->locks[level] != BTRFS_WRITE_LOCK_BLOCKING);
         WARN_ON(btrfs_header_generation(mid) != trans->transid);
 
         orig_ptr = btrfs_node_blockptr(mid, orig_slot);
@@ -1360,7 +1395,7 @@ static noinline void unlock_up(struct btrfs_path *path, int level,
 
                 t = path->nodes[i];
                 if (i >= lowest_unlock && i > skip_level && path->locks[i]) {
-                        btrfs_tree_unlock(t);
+                        btrfs_tree_unlock_rw(t, path->locks[i]);
                         path->locks[i] = 0;
                 }
         }
@@ -1387,7 +1422,7 @@ noinline void btrfs_unlock_up_safe(struct btrfs_path *path, int level)
                         continue;
                 if (!path->locks[i])
                         continue;
-                btrfs_tree_unlock(path->nodes[i]);
+                btrfs_tree_unlock_rw(path->nodes[i], path->locks[i]);
                 path->locks[i] = 0;
         }
 }
@@ -1436,6 +1471,8 @@ read_block_for_search(struct btrfs_trans_handle *trans,
                          * we can trust our generation number
                          */
                         free_extent_buffer(tmp);
+                        btrfs_set_path_blocking(p);
+
                         tmp = read_tree_block(root, blocknr, blocksize, gen);
                         if (tmp && btrfs_buffer_uptodate(tmp, gen)) {
                                 *eb_ret = tmp;
@@ -1491,20 +1528,27 @@ read_block_for_search(struct btrfs_trans_handle *trans,
 static int
 setup_nodes_for_search(struct btrfs_trans_handle *trans,
                        struct btrfs_root *root, struct btrfs_path *p,
-                       struct extent_buffer *b, int level, int ins_len)
+                       struct extent_buffer *b, int level, int ins_len,
+                       int *write_lock_level)
 {
         int ret;
         if ((p->search_for_split || ins_len > 0) && btrfs_header_nritems(b) >=
             BTRFS_NODEPTRS_PER_BLOCK(root) - 3) {
                 int sret;
 
+                if (*write_lock_level < level + 1) {
+                        *write_lock_level = level + 1;
+                        btrfs_release_path(p);
+                        goto again;
+                }
+
                 sret = reada_for_balance(root, p, level);
                 if (sret)
                         goto again;
 
                 btrfs_set_path_blocking(p);
                 sret = split_node(trans, root, p, level);
-                btrfs_clear_path_blocking(p, NULL);
+                btrfs_clear_path_blocking(p, NULL, 0);
 
                 BUG_ON(sret > 0);
                 if (sret) {
@@ -1516,13 +1560,19 @@ setup_nodes_for_search(struct btrfs_trans_handle *trans,
                    BTRFS_NODEPTRS_PER_BLOCK(root) / 2) {
                 int sret;
 
+                if (*write_lock_level < level + 1) {
+                        *write_lock_level = level + 1;
+                        btrfs_release_path(p);
+                        goto again;
+                }
+
                 sret = reada_for_balance(root, p, level);
                 if (sret)
                         goto again;
 
                 btrfs_set_path_blocking(p);
                 sret = balance_level(trans, root, p, level);
-                btrfs_clear_path_blocking(p, NULL);
+                btrfs_clear_path_blocking(p, NULL, 0);
 
                 if (sret) {
                         ret = sret;
@@ -1566,27 +1616,78 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root
         int err;
         int level;
         int lowest_unlock = 1;
+        int root_lock;
+        /* everything at write_lock_level or lower must be write locked */
+        int write_lock_level = 0;
         u8 lowest_level = 0;
 
         lowest_level = p->lowest_level;
         WARN_ON(lowest_level && ins_len > 0);
         WARN_ON(p->nodes[0] != NULL);
 
-        if (ins_len < 0)
+        if (ins_len < 0) {
                 lowest_unlock = 2;
 
+                /* when we are removing items, we might have to go up to level
+                 * two as we update tree pointers  Make sure we keep write
+                 * for those levels as well
+                 */
+                write_lock_level = 2;
+        } else if (ins_len > 0) {
+                /*
+                 * for inserting items, make sure we have a write lock on
+                 * level 1 so we can update keys
+                 */
+                write_lock_level = 1;
+        }
+
+        if (!cow)
+                write_lock_level = -1;
+
+        if (cow && (p->keep_locks || p->lowest_level))
+                write_lock_level = BTRFS_MAX_LEVEL;
+
 again:
+        /*
+         * we try very hard to do read locks on the root
+         */
+        root_lock = BTRFS_READ_LOCK;
+        level = 0;
         if (p->search_commit_root) {
+                /*
+                 * the commit roots are read only
+                 * so we always do read locks
+                 */
                 b = root->commit_root;
                 extent_buffer_get(b);
+                level = btrfs_header_level(b);
                 if (!p->skip_locking)
-                        btrfs_tree_lock(b);
+                        btrfs_tree_read_lock(b);
         } else {
-                if (p->skip_locking)
+                if (p->skip_locking) {
                         b = btrfs_root_node(root);
-                else
-                        b = btrfs_lock_root_node(root);
+                        level = btrfs_header_level(b);
+                } else {
+                        /* we don't know the level of the root node
+                         * until we actually have it read locked
+                         */
+                        b = btrfs_read_lock_root_node(root);
+                        level = btrfs_header_level(b);
+                        if (level <= write_lock_level) {
+                                /* whoops, must trade for write lock */
+                                btrfs_tree_read_unlock(b);
+                                free_extent_buffer(b);
+                                b = btrfs_lock_root_node(root);
+                                root_lock = BTRFS_WRITE_LOCK;
+
+                                /* the level might have changed, check again */
+                                level = btrfs_header_level(b);
+                        }
+                }
         }
+        p->nodes[level] = b;
+        if (!p->skip_locking)
+                p->locks[level] = root_lock;
 
         while (b) {
                 level = btrfs_header_level(b);
@@ -1595,10 +1696,6 @@ again:
                  * setup the path here so we can release it under lock
                  * contention with the cow code
                  */
-                p->nodes[level] = b;
-                if (!p->skip_locking)
-                        p->locks[level] = 1;
-
                 if (cow) {
                         /*
                          * if we don't really need to cow this block
@@ -1610,6 +1707,16 @@ again:
 
                         btrfs_set_path_blocking(p);
 
+                        /*
+                         * must have write locks on this node and the
+                         * parent
+                         */
+                        if (level + 1 > write_lock_level) {
+                                write_lock_level = level + 1;
+                                btrfs_release_path(p);
+                                goto again;
+                        }
+
                         err = btrfs_cow_block(trans, root, b,
                                               p->nodes[level + 1],
                                               p->slots[level + 1], &b);
@@ -1622,10 +1729,7 @@ cow_done:
                 BUG_ON(!cow && ins_len);
 
                 p->nodes[level] = b;
-                if (!p->skip_locking)
-                        p->locks[level] = 1;
-
-                btrfs_clear_path_blocking(p, NULL);
+                btrfs_clear_path_blocking(p, NULL, 0);
 
                 /*
                  * we have a lock on b and as long as we aren't changing
@@ -1651,7 +1755,7 @@ cow_done:
                         }
                         p->slots[level] = slot;
                         err = setup_nodes_for_search(trans, root, p, b, level,
-                                                     ins_len);
+                                             ins_len, &write_lock_level);
                         if (err == -EAGAIN)
                                 goto again;
                         if (err) {
@@ -1661,6 +1765,19 @@ cow_done:
                         b = p->nodes[level];
                         slot = p->slots[level];
 
+                        /*
+                         * slot 0 is special, if we change the key
+                         * we have to update the parent pointer
+                         * which means we must have a write lock
+                         * on the parent
+                         */
+                        if (slot == 0 && cow &&
+                            write_lock_level < level + 1) {
+                                write_lock_level = level + 1;
+                                btrfs_release_path(p);
+                                goto again;
+                        }
+
                         unlock_up(p, level, lowest_unlock);
 
                         if (level == lowest_level) {
@@ -1679,23 +1796,42 @@ cow_done:
                         }
 
                         if (!p->skip_locking) {
-                                btrfs_clear_path_blocking(p, NULL);
-                                err = btrfs_try_spin_lock(b);
-
-                                if (!err) {
-                                        btrfs_set_path_blocking(p);
-                                        btrfs_tree_lock(b);
-                                        btrfs_clear_path_blocking(p, b);
+                                level = btrfs_header_level(b);
+                                if (level <= write_lock_level) {
+                                        err = btrfs_try_tree_write_lock(b);
+                                        if (!err) {
+                                                btrfs_set_path_blocking(p);
+                                                btrfs_tree_lock(b);
+                                                btrfs_clear_path_blocking(p, b,
+                                                                  BTRFS_WRITE_LOCK);
+                                        }
+                                        p->locks[level] = BTRFS_WRITE_LOCK;
+                                } else {
+                                        err = btrfs_try_tree_read_lock(b);
+                                        if (!err) {
+                                                btrfs_set_path_blocking(p);
+                                                btrfs_tree_read_lock(b);
+                                                btrfs_clear_path_blocking(p, b,
+                                                                  BTRFS_READ_LOCK);
+                                        }
+                                        p->locks[level] = BTRFS_READ_LOCK;
                                 }
+                                p->nodes[level] = b;
                         }
                 } else {
                         p->slots[level] = slot;
                         if (ins_len > 0 &&
                             btrfs_leaf_free_space(root, b) < ins_len) {
+                                if (write_lock_level < 1) {
+                                        write_lock_level = 1;
+                                        btrfs_release_path(p);
+                                        goto again;
+                                }
+
                                 btrfs_set_path_blocking(p);
                                 err = split_leaf(trans, root, key,
                                                  p, ins_len, ret == 0);
-                                btrfs_clear_path_blocking(p, NULL);
+                                btrfs_clear_path_blocking(p, NULL, 0);
 
                                 BUG_ON(err > 0);
                                 if (err) {
@@ -1976,7 +2112,7 @@ static noinline int insert_new_root(struct btrfs_trans_handle *trans,
         add_root_to_dirty_list(root);
         extent_buffer_get(c);
         path->nodes[level] = c;
-        path->locks[level] = 1;
+        path->locks[level] = BTRFS_WRITE_LOCK;
         path->slots[level] = 0;
         return 0;
 }
@@ -3819,11 +3955,11 @@ int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
 
         WARN_ON(!path->keep_locks);
 again:
-        cur = btrfs_lock_root_node(root);
+        cur = btrfs_read_lock_root_node(root);
         level = btrfs_header_level(cur);
         WARN_ON(path->nodes[level]);
         path->nodes[level] = cur;
-        path->locks[level] = 1;
+        path->locks[level] = BTRFS_READ_LOCK;
 
         if (btrfs_header_generation(cur) < min_trans) {
                 ret = 1;
@@ -3913,12 +4049,12 @@ find_next_key:
                 cur = read_node_slot(root, cur, slot);
                 BUG_ON(!cur);
 
-                btrfs_tree_lock(cur);
+                btrfs_tree_read_lock(cur);
 
-                path->locks[level - 1] = 1;
+                path->locks[level - 1] = BTRFS_READ_LOCK;
                 path->nodes[level - 1] = cur;
                 unlock_up(path, level, 1);
-                btrfs_clear_path_blocking(path, NULL);
+                btrfs_clear_path_blocking(path, NULL, 0);
         }
 out:
         if (ret == 0)
@@ -4034,6 +4170,7 @@ int btrfs_next_leaf(struct btrfs_root *root, struct btrfs_path *path)
         int ret;
         int old_spinning = path->leave_spinning;
         int force_blocking = 0;
+        int next_rw_lock = 0;
 
         nritems = btrfs_header_nritems(path->nodes[0]);
         if (nritems == 0)
@@ -4051,6 +4188,7 @@ int btrfs_next_leaf(struct btrfs_root *root, struct btrfs_path *path)
 again:
         level = 1;
         next = NULL;
+        next_rw_lock = 0;
         btrfs_release_path(path);
 
         path->keep_locks = 1;
@@ -4096,11 +4234,12 @@ again:
                 }
 
                 if (next) {
-                        btrfs_tree_unlock(next);
+                        btrfs_tree_unlock_rw(next, next_rw_lock);
                         free_extent_buffer(next);
                 }
 
                 next = c;
+                next_rw_lock = path->locks[level];
                 ret = read_block_for_search(NULL, root, path, &next, level,
                                             slot, &key);
                 if (ret == -EAGAIN)
@@ -4112,15 +4251,22 @@ again:
                 }
 
                 if (!path->skip_locking) {
-                        ret = btrfs_try_spin_lock(next);
+                        ret = btrfs_try_tree_read_lock(next);
                         if (!ret) {
                                 btrfs_set_path_blocking(path);
-                                btrfs_tree_lock(next);
-                                if (!force_blocking)
-                                        btrfs_clear_path_blocking(path, next);
+                                btrfs_tree_read_lock(next);
+                                if (!force_blocking) {
+                                        btrfs_clear_path_blocking(path, next,
+                                                          BTRFS_READ_LOCK);
+                                }
+                        }
+                        if (force_blocking) {
+                                btrfs_set_lock_blocking_rw(next,
+                                                           BTRFS_READ_LOCK);
+                                next_rw_lock = BTRFS_READ_LOCK_BLOCKING;
+                        } else {
+                                next_rw_lock = BTRFS_READ_LOCK;
                         }
-                        if (force_blocking)
-                                btrfs_set_lock_blocking(next);
                 }
                 break;
         }
@@ -4129,14 +4275,13 @@ again:
                 level--;
                 c = path->nodes[level];
                 if (path->locks[level])
-                        btrfs_tree_unlock(c);
+                        btrfs_tree_unlock_rw(c, path->locks[level]);
 
                 free_extent_buffer(c);
                 path->nodes[level] = next;
                 path->slots[level] = 0;
                 if (!path->skip_locking)
-                        path->locks[level] = 1;
-
+                        path->locks[level] = next_rw_lock;
                 if (!level)
                         break;
 
@@ -4151,16 +4296,21 @@ again:
                 }
 
                 if (!path->skip_locking) {
-                        btrfs_assert_tree_locked(path->nodes[level]);
-                        ret = btrfs_try_spin_lock(next);
+                        ret = btrfs_try_tree_read_lock(next);
                         if (!ret) {
                                 btrfs_set_path_blocking(path);
-                                btrfs_tree_lock(next);
+                                btrfs_tree_read_lock(next);
                                 if (!force_blocking)
-                                        btrfs_clear_path_blocking(path, next);
+                                        btrfs_clear_path_blocking(path, next,
+                                                          BTRFS_READ_LOCK);
+                        }
+                        if (force_blocking) {
+                                btrfs_set_lock_blocking_rw(next,
+                                                   BTRFS_READ_LOCK);
+                                next_rw_lock = BTRFS_READ_LOCK_BLOCKING;
+                        } else {
+                                next_rw_lock = BTRFS_READ_LOCK;
                         }
-                        if (force_blocking)
-                                btrfs_set_lock_blocking(next);
                 }
         }
         ret = 0;