writeback: add lockdep annotation to inode_to_wb()

With the previous three patches, all operations which acquire wb from
inode are either under one of inode->i_lock, mapping->tree_lock or
wb->list_lock or protected by unlocked_inode_to_wb transaction.  This
will be depended upon by foreign inode wb switching.

This patch adds lockdep assertion to inode_to_wb() so that usages
outside the above list locks can be caught easily.  There are three
exceptions.

* locked_inode_to_wb_and_lock_list() is holding wb->list_lock but the
  wb may not be the inode's.  Ensuring that is the function's role
  after all.  Updated to deref inode->i_wb directly.

* inode_wb_stat_unlocked_begin() is usually protected by combination
  of !I_WB_SWITCH and rcu_read_lock().  Updated to deref inode->i_wb
  directly.

* inode_congested() wants to test whether inode->i_wb is set before
  starting the transaction.  Added inode_to_wb_is_valid() which tests
  inode->i_wb directly.

v5: might_lock() removed.  It annotates that the lock is grabbed w/
    irq enabled which isn't the case and triggering lockdep warning
    spuriously.

v4: might_lock() added to unlocked_inode_to_wb_begin().

v3: inode_congested() conversion added.

v2: locked_inode_to_wb_and_lock_list() was missing in the first
    version.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 25458fa4753d..6b99deeb9de0 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -285,7 +285,8 @@ locked_inode_to_wb_and_lock_list(struct inode *inode)
 		spin_lock(&wb->list_lock);
 		wb_put(wb);		/* not gonna deref it anymore */
 
-		if (likely(wb == inode_to_wb(inode)))
+		/* i_wb may have changed inbetween, can't use inode_to_wb() */
+		if (likely(wb == inode->i_wb))
 			return wb;	/* @inode already has ref */
 
 		spin_unlock(&wb->list_lock);
@@ -622,7 +623,7 @@ int inode_congested(struct inode *inode, int cong_bits)
 	 * Once set, ->i_wb never becomes NULL while the inode is alive.
 	 * Start transaction iff ->i_wb is visible.
 	 */
-	if (inode && inode_to_wb(inode)) {
+	if (inode && inode_to_wb_is_valid(inode)) {
 		struct bdi_writeback *wb;
 		bool locked, congested;