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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>
This commit is contained in:
Jiufei Xue 2019-05-17 14:31:44 -07:00 committed by Linus Torvalds
parent 60fce36afa
commit ec084de929
1 changed files with 8 additions and 3 deletions
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11
fs/fs-writeback.c
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@ -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 @@ restart:
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);
}
}