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

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Message-Id: <1466022910-11243-1-git-send-email-tahsin@google.com>
Date:	Wed, 15 Jun 2016 13:35:10 -0700
From:	Tahsin Erdogan <tahsin@...gle.com>
To:	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Tahsin Erdogan <tahsin@...gle.com>
Subject: [PATCH v2] writeback: inode cgroup wb switch should not call ihold()

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@...nel.org>
Signed-off-by: Tahsin Erdogan <tahsin@...gle.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