writeback: Move clearing of I_SYNC into inode_sync_complete()

Move clearing of I_SYNC into inode_sync_complete().  It is more logical to have
clearing of I_SYNC bit and waking of waiters in one place. Also later we will
have two places needing to clear I_SYNC and wake up waiters so this allows them
to use the common helper. Moving of I_SYNC clearing to a later stage of
writeback_single_inode() is safe since we hold i_lock all the time.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>

fs/fs-writeback.c

@@ -231,11 +231,8 @@ static void requeue_io(struct inode *inode, struct bdi_writeback *wb)
 
 static void inode_sync_complete(struct inode *inode)
 {
-	/*
-	 * Prevent speculative execution through
-	 * spin_unlock(&wb->list_lock);
-	 */
-
+	inode->i_state &= ~I_SYNC;
+	/* Waiters must see I_SYNC cleared before being woken up */
 	smp_mb();
 	wake_up_bit(&inode->i_state, __I_SYNC);
 }
@@ -436,7 +433,6 @@ writeback_single_inode(struct inode *inode, struct bdi_writeback *wb,
 
 	spin_lock(&wb->list_lock);
 	spin_lock(&inode->i_lock);
-	inode->i_state &= ~I_SYNC;
 	if (!(inode->i_state & I_FREEING)) {
 		/*
 		 * Sync livelock prevention. Each inode is tagged and synced in