[PATCH 1/1] writeback fix bdi thread race in mark_inode_dirty

[Srinivas Eeda <srinivas.eeda@oracle.com>]

In __mark_inode_dirty, a process checks !wb_has_dirty_io outside of list_lock
spinlock. This could cause a race, where process sees that list has dirty io
and decides not wake up bdi thread and waits for spinlock to add to dirty list.
Right at this time bdi_writeback_workfn finished write-back on last inode.
It sees the list is empty and ends. Process could now get the spinlock and
add inode to dirty list and doesn't wakeup bdi thread. Future calls to
__mark_inode_dirty also do not wake up the thread because list is not empty
any more.

Fix is to get wb.list_lock spinlock before checking the dirty list

Signed-off-by: Srinivas Eeda <srinivas.eeda@oracle.com>
---
 fs/fs-writeback.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 68851ff..a5abc6c 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1173,6 +1173,9 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 			bool wakeup_bdi = false;
 			bdi = inode_to_bdi(inode);
 
+			spin_unlock(&inode->i_lock);
+			spin_lock(&bdi->wb.list_lock);
+
 			if (bdi_cap_writeback_dirty(bdi)) {
 				WARN(!test_bit(BDI_registered, &bdi->state),
 				     "bdi-%s not registered\n", bdi->name);
@@ -1187,8 +1190,6 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 					wakeup_bdi = true;
 			}
 
-			spin_unlock(&inode->i_lock);
-			spin_lock(&bdi->wb.list_lock);
 			inode->dirtied_when = jiffies;
 			list_move(&inode->i_wb_list, &bdi->wb.b_dirty);
 			spin_unlock(&bdi->wb.list_lock);
-- 
1.5.6.5