fs/writeback.c: use rcu_barrier() to wait for inflight wb switches going into workqueue when umount
synchronize_rcu() didn't wait for call_rcu() callbacks, so inode wb
switch may not go to the workqueue after synchronize_rcu(). Thus
previous scheduled switches was not finished even flushing the
workqueue, which will cause a NULL pointer dereferenced followed below.
VFS: Busy inodes after unmount of vdd. Self-destruct in 5 seconds. Have a nice day...
BUG: unable to handle kernel NULL pointer dereference at 0000000000000278
evict+0xb3/0x180
iput+0x1b0/0x230
inode_switch_wbs_work_fn+0x3c0/0x6a0
worker_thread+0x4e/0x490
? process_one_work+0x410/0x410
kthread+0xe6/0x100
ret_from_fork+0x39/0x50
Replace the synchronize_rcu() call with a rcu_barrier() to wait for all
pending callbacks to finish. And inc isw_nr_in_flight after call_rcu()
in inode_switch_wbs() to make more sense.
Link: http://lkml.kernel.org/r/20190429024108.54150-1-jiufei.xue@linux.alibaba.com
Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Acked-by: Tejun Heo <tj@kernel.org>
Suggested-by: Tejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 36855c1..b16645b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -523,8 +523,6 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
isw->inode = inode;
- atomic_inc(&isw_nr_in_flight);
-
/*
* In addition to synchronizing among switchers, I_WB_SWITCH tells
* the RCU protected stat update paths to grab the i_page
@@ -532,6 +530,9 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
* Let's continue after I_WB_SWITCH is guaranteed to be visible.
*/
call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
+
+ atomic_inc(&isw_nr_in_flight);
+
goto out_unlock;
out_free:
@@ -901,7 +902,11 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
void cgroup_writeback_umount(void)
{
if (atomic_read(&isw_nr_in_flight)) {
- synchronize_rcu();
+ /*
+ * Use rcu_barrier() to wait for all pending callbacks to
+ * ensure that all in-flight wb switches are in the workqueue.
+ */
+ rcu_barrier();
flush_workqueue(isw_wq);
}
}
