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? --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
