[PATCH v2] writeback: inode cgroup wb switch should not call ihold()

[PATCH v2] writeback: inode cgroup wb switch should not call ihold()

[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, anyone calling ihold() must already have a ref count on inode,
but in this case inode->i_count may already be zero:

------------[ 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 ]---

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tahsin Erdogan <tahsin@google.com>
---
v2:
 removed inode->i_count == 0 check
 replaced ihold() with __iget()
 updated commit description

 fs/fs-writeback.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 989a2cef6b76..fe7e83a45eff 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -483,9 +483,9 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 		goto out_free;
 	}
 	inode->i_state |= I_WB_SWITCH;
+	__iget(inode);
 	spin_unlock(&inode->i_lock);
 
-	ihold(inode);
 	isw->inode = inode;
 
 	atomic_inc(&isw_nr_in_flight);
-- 
2.8.0.rc3.226.g39d4020

Re: [PATCH v2] writeback: inode cgroup wb switch should not call ihold()

[Jan Kara]

On Wed 15-06-16 13:35:10, Tahsin Erdogan wrote:
> Asynchronous wb switching of inodes takes an additional ref count on an
> inode to make sure inode remains valid until switchover is completed.
> 
> However, anyone calling ihold() must already have a ref count on inode,
> but in this case inode->i_count may already be zero:
> 
> ------------[ 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 ]---
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Tahsin Erdogan <tahsin@google.com>

Looks good. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH v2] writeback: inode cgroup wb switch should not call ihold()

[Tahsin Erdogan]

On Thu, Jun 16, 2016 at 12:13 AM, Jan Kara <jack@suse.cz> wrote:
> Looks good. You can add:
>
> Reviewed-by: Jan Kara <jack@suse.cz>

Thanks Jan and Tejun!