From 857bc13f857aea957ae038b2b43c28560976024a Mon Sep 17 00:00:00 2001 From: Josef Bacik Date: Mon, 12 Sep 2022 12:27:42 -0700 Subject: [PATCH] btrfs: implement a nowait option for tree searches For NOWAIT IOCBs we'll need a way to tell search to not wait on locks or anything. Accomplish this by adding a path->nowait flag that will use trylocks and skip reading of metadata, returning -EAGAIN in either of these cases. For now we only need this for reads, so only the read side is handled. Add an ASSERT() to catch anybody trying to use this for writes so they know they'll have to implement the write side. Reviewed-by: Filipe Manana Signed-off-by: Josef Bacik Signed-off-by: Stefan Roesch Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/ctree.c | 39 ++++++++++++++++++++++++++++++++++++--- fs/btrfs/ctree.h | 2 ++ fs/btrfs/locking.c | 25 +++++++++++++++++++++++++ fs/btrfs/locking.h | 1 + 4 files changed, 64 insertions(+), 3 deletions(-) diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c index ebfa35fe1c38..84548b4074f7 100644 --- a/fs/btrfs/ctree.c +++ b/fs/btrfs/ctree.c @@ -1447,6 +1447,11 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, return 0; } + if (p->nowait) { + free_extent_buffer(tmp); + return -EAGAIN; + } + if (unlock_up) btrfs_unlock_up_safe(p, level + 1); @@ -1467,6 +1472,8 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, ret = -EAGAIN; goto out; + } else if (p->nowait) { + return -EAGAIN; } if (unlock_up) { @@ -1634,7 +1641,13 @@ static struct extent_buffer *btrfs_search_slot_get_root(struct btrfs_root *root, * We don't know the level of the root node until we actually * have it read locked */ - b = btrfs_read_lock_root_node(root); + if (p->nowait) { + b = btrfs_try_read_lock_root_node(root); + if (IS_ERR(b)) + return b; + } else { + b = btrfs_read_lock_root_node(root); + } level = btrfs_header_level(b); if (level > write_lock_level) goto out; @@ -1910,6 +1923,13 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root, WARN_ON(p->nodes[0] != NULL); BUG_ON(!cow && ins_len); + /* + * For now only allow nowait for read only operations. There's no + * strict reason why we can't, we just only need it for reads so it's + * only implemented for reads. + */ + ASSERT(!p->nowait || !cow); + if (ins_len < 0) { lowest_unlock = 2; @@ -1936,7 +1956,12 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root, if (p->need_commit_sem) { ASSERT(p->search_commit_root); - down_read(&fs_info->commit_root_sem); + if (p->nowait) { + if (!down_read_trylock(&fs_info->commit_root_sem)) + return -EAGAIN; + } else { + down_read(&fs_info->commit_root_sem); + } } again: @@ -2082,7 +2107,15 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root, btrfs_tree_lock(b); p->locks[level] = BTRFS_WRITE_LOCK; } else { - btrfs_tree_read_lock(b); + if (p->nowait) { + if (!btrfs_try_tree_read_lock(b)) { + free_extent_buffer(b); + ret = -EAGAIN; + goto done; + } + } else { + btrfs_tree_read_lock(b); + } p->locks[level] = BTRFS_READ_LOCK; } p->nodes[level] = b; diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h index 38c4c4fa2762..17af7e317e8e 100644 --- a/fs/btrfs/ctree.h +++ b/fs/btrfs/ctree.h @@ -443,6 +443,8 @@ struct btrfs_path { * header (ie. sizeof(struct btrfs_item) is not included). */ unsigned int search_for_extension:1; + /* Stop search if any locks need to be taken (for read) */ + unsigned int nowait:1; }; struct btrfs_dev_replace { diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c index 9063072b399b..0eab3cb274a1 100644 --- a/fs/btrfs/locking.c +++ b/fs/btrfs/locking.c @@ -285,6 +285,31 @@ struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root) return eb; } +/* + * Loop around taking references on and locking the root node of the tree in + * nowait mode until we end up with a lock on the root node or returning to + * avoid blocking. + * + * Return: root extent buffer with read lock held or -EAGAIN. + */ +struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root) +{ + struct extent_buffer *eb; + + while (1) { + eb = btrfs_root_node(root); + if (!btrfs_try_tree_read_lock(eb)) { + free_extent_buffer(eb); + return ERR_PTR(-EAGAIN); + } + if (eb == root->node) + break; + btrfs_tree_read_unlock(eb); + free_extent_buffer(eb); + } + return eb; +} + /* * DREW locks * ========== diff --git a/fs/btrfs/locking.h b/fs/btrfs/locking.h index ab268be09bb5..490c7a79e995 100644 --- a/fs/btrfs/locking.h +++ b/fs/btrfs/locking.h @@ -94,6 +94,7 @@ int btrfs_try_tree_read_lock(struct extent_buffer *eb); int btrfs_try_tree_write_lock(struct extent_buffer *eb); struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root); struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root); +struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root); #ifdef CONFIG_BTRFS_DEBUG static inline void btrfs_assert_tree_write_locked(struct extent_buffer *eb)