[PATCH] writeback: inode cgroup wb switch should skip inode with zero i_count

[Tahsin Erdogan]

Asynchronous wb switching of inodes takes an additional ref count on an
inode to make sure inode remains valid until switchover is completed.

However, it is possible that inode->i_count has already reached zero
while inode is in writeback queue:

------------[ cut here ]------------
WARNING: CPU: 1 PID: 917 at fs/inode.c:397 ihold+0x2b/0x30
CPU: 1 PID: 917 Comm: kworker/u4:5 Not tainted 4.7.0-rc2+ #49
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
01/01/2011
Workqueue: writeback wb_workfn (flush-8:16)
 0000000000000000 ffff88007ca0fb58 ffffffff805990af 0000000000000000
 0000000000000000 ffff88007ca0fb98 ffffffff80268702 0000018d000004e2
 ffff88007cef40e8 ffff88007c9b89a8 ffff880079e3a740 0000000000000003
Call Trace:
 [<ffffffff805990af>] dump_stack+0x4d/0x6e
 [<ffffffff80268702>] __warn+0xc2/0xe0
 [<ffffffff802687d8>] warn_slowpath_null+0x18/0x20
 [<ffffffff8035b4ab>] ihold+0x2b/0x30
 [<ffffffff80367ecc>] inode_switch_wbs+0x11c/0x180
 [<ffffffff80369110>] wbc_detach_inode+0x170/0x1a0
 [<ffffffff80369abc>] writeback_sb_inodes+0x21c/0x530
 [<ffffffff80369f7e>] wb_writeback+0xee/0x1e0
 [<ffffffff8036a147>] wb_workfn+0xd7/0x280
 [<ffffffff80287531>] ? try_to_wake_up+0x1b1/0x2b0
 [<ffffffff8027bb09>] process_one_work+0x129/0x300
 [<ffffffff8027be06>] worker_thread+0x126/0x480
 [<ffffffff8098cde7>] ? __schedule+0x1c7/0x561
 [<ffffffff8027bce0>] ? process_one_work+0x300/0x300
 [<ffffffff80280ff4>] kthread+0xc4/0xe0
 [<ffffffff80335578>] ? kfree+0xc8/0x100
 [<ffffffff809903cf>] ret_from_fork+0x1f/0x40
 [<ffffffff80280f30>] ? __kthread_parkme+0x70/0x70
---[ end trace aaefd2fd9f306bc4 ]---

Signed-off-by: Tahsin Erdogan <tahsin@xxxxxxxxxx>
---
 fs/fs-writeback.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 989a2ce..b44ede0 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -478,14 +478,15 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	spin_lock(&inode->i_lock);
 	if (!(inode->i_sb->s_flags & MS_ACTIVE) ||
 	    inode->i_state & (I_WB_SWITCH | I_FREEING) ||
+	    atomic_read(&inode->i_count) == 0 ||
 	    inode_to_wb(inode) == isw->new_wb) {
 		spin_unlock(&inode->i_lock);
 		goto out_free;
 	}
 	inode->i_state |= I_WB_SWITCH;
+	ihold(inode);
 	spin_unlock(&inode->i_lock);
 
-	ihold(inode);
 	isw->inode = inode;
 
 	atomic_inc(&isw_nr_in_flight);
-- 
2.8.0.rc3.226.g39d4020