Hi Darrick, On 2017/9/19 2:52, Darrick J. Wong wrote: > 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@xxxxxx> >> Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx> >> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx> > > 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? It's difficult to create an always-happened test for the race, and I had test the patch by adding artificial delays (as suggested by Brian Foster) in kernel source code. I also have tried to reproduce the problem by lifting up the schedule class/priority of the umount process and lifting down the schedule class/priorityof the xfsaild kthread, but still can not reproduce the problem, so any ideas or suggestions ? Regard > --D > >> --- >> 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@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > . > -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
