Re: [PATCH v2] xfs: check kthread_should_stop() after the setting of task state
From: Darrick J. Wong <hidden>
Date: 2017-09-18 18:52:07
On Tue, Sep 12, 2017 at 04:45:30PM +0800, Hou Tao wrote:
A umount hang is possible when a race occurs between the umount
process and the xfsaild kthread. The following sequences outline
the race:
xfsaild: kthread_should_stop()
=> return false, so xfsaild continue
umount: set_bit(KTHREAD_SHOULD_STOP, &kthread->flags)
=> by kthread_stop()
umount: wake_up_process()
=> because xfsaild is still running, so 0 is returned
xfsaild: __set_current_state(TASK_INTERRUPTIBLE)
xfsaild: schedule()
=> now, xfsaild will wait indefinitely
umount: wait_for_completion()
=> and umount will hang
To fix that, we need to check kthread_should_stop() after we set
the task state, so the xfsaild will either see the stop bit and
exit or the task state is reset to runnable by wake_up_process()
such that it isn't scheduled out indefinitely and detects the stop
bit at the next iteration.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <redacted>
Signed-off-by: Hou Tao <redacted>Looks ok, I guess, but just to reiterate what I said in another email 15 minutes ago, can this be turned into a regression test? --D
quoted hunk ↗ jump to hunk
--- v2: * comment updates suggested by Brain Foster v1: * http://www.spinics.net/lists/linux-xfs/msg10285.html --- fs/xfs/xfs_trans_ail.c | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-)diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c index 9056c0f..2d77d9c 100644 --- a/fs/xfs/xfs_trans_ail.c +++ b/fs/xfs/xfs_trans_ail.c@@ -499,11 +499,26 @@ xfsaild( current->flags |= PF_MEMALLOC; set_freezable(); - while (!kthread_should_stop()) { + while (1) { if (tout && tout <= 20) - __set_current_state(TASK_KILLABLE); + set_current_state(TASK_KILLABLE); else - __set_current_state(TASK_INTERRUPTIBLE); + set_current_state(TASK_INTERRUPTIBLE); + + /* + * Check kthread_should_stop() after we set the task state + * to guarantee that we either see the stop bit and exit or + * the task state is reset to runnable such that it's not + * scheduled out indefinitely and detects the stop bit at + * next iteration. + * + * A memory barrier is included in above task state set to + * serialize again kthread_stop(). + */ + if (kthread_should_stop()) { + __set_current_state(TASK_RUNNING); + break; + } spin_lock(&ailp->xa_lock);-- 2.5.0 -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html