Commit f658822c authored by Jiufei Xue's avatar Jiufei Xue Committed by Greg Kroah-Hartman

fs/writeback.c: use rcu_barrier() to wait for inflight wb switches going into workqueue when umount

commit ec084de9 upstream.

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.comSigned-off-by: default avatarJiufei Xue <jiufei.xue@linux.alibaba.com>
Acked-by: default avatarTejun Heo <tj@kernel.org>
Suggested-by: default avatarTejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 691306eb
......@@ -530,8 +530,6 @@ 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 mapping's
......@@ -539,6 +537,9 @@ 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:
......@@ -908,7 +909,11 @@ 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);
}
}
......
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