Re: [PATCH 4/8] md-cluster: introduce dlm_lock_sync_interruptible to fix tasks hang

[Shaohua Li <shli@kernel.org>]

On Mon, Aug 01, 2016 at 11:24:34PM -0400, Guoqing Jiang wrote:
> 
> 
> On 08/01/2016 06:20 PM, Shaohua Li wrote:
> >On Thu, Jul 28, 2016 at 02:16:48AM -0400, Guoqing Jiang wrote:
> >>When some node leaves cluster, then it's bitmap need to be
> >>synced by another node, so "md*_recover" thread is triggered
> >>for the purpose. However, with below steps. we can find tasks
> >>hang happened either in B or C.
> >>
> >>1. Node A create a resyncing cluster raid1, assemble it in
> >>    other two nodes (B and C).
> >>2. stop array in B and C.
> >>3. stop array in A.
> >>
> >>linux44:~ # ps aux|grep md|grep D
> >>root	5938	0.0  0.1  19852  1964 pts/0    D+   14:52   0:00 mdadm -S md0
> >>root	5939	0.0  0.0      0     0 ?        D    14:52   0:00 [md0_recover]
> >>
> >>linux44:~ # cat /proc/5939/stack
> >>[<ffffffffa04cf321>] dlm_lock_sync+0x71/0x90 [md_cluster]
> >>[<ffffffffa04d0705>] recover_bitmaps+0x125/0x220 [md_cluster]
> >>[<ffffffffa052105d>] md_thread+0x16d/0x180 [md_mod]
> >>[<ffffffff8107ad94>] kthread+0xb4/0xc0
> >>[<ffffffff8152a518>] ret_from_fork+0x58/0x90
> >>
> >>linux44:~ # cat /proc/5938/stack
> >>[<ffffffff8107afde>] kthread_stop+0x6e/0x120
> >>[<ffffffffa0519da0>] md_unregister_thread+0x40/0x80 [md_mod]
> >>[<ffffffffa04cfd20>] leave+0x70/0x120 [md_cluster]
> >>[<ffffffffa0525e24>] md_cluster_stop+0x14/0x30 [md_mod]
> >>[<ffffffffa05269ab>] bitmap_free+0x14b/0x150 [md_mod]
> >>[<ffffffffa0523f3b>] do_md_stop+0x35b/0x5a0 [md_mod]
> >>[<ffffffffa0524e83>] md_ioctl+0x873/0x1590 [md_mod]
> >>[<ffffffff81288464>] blkdev_ioctl+0x214/0x7d0
> >>[<ffffffff811dd3dd>] block_ioctl+0x3d/0x40
> >>[<ffffffff811b92d4>] do_vfs_ioctl+0x2d4/0x4b0
> >>[<ffffffff811b9538>] SyS_ioctl+0x88/0xa0
> >>[<ffffffff8152a5c9>] system_call_fastpath+0x16/0x1b
> >>
> >>The problem is caused by recover_bitmaps can't reliably abort
> >>when the thread is unregistered. So dlm_lock_sync_interruptible
> >>is introduced to detect the thread's situation to fix the problem.
> >>
> >>Reviewed-by: NeilBrown <neilb@suse.com>
> >>Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
> >>---
> >>  drivers/md/md-cluster.c | 38 +++++++++++++++++++++++++++++++++++++-
> >>  1 file changed, 37 insertions(+), 1 deletion(-)
> >>
> >>diff --git a/drivers/md/md-cluster.c b/drivers/md/md-cluster.c
> >>index ea2699e..f3d584e 100644
> >>--- a/drivers/md/md-cluster.c
> >>+++ b/drivers/md/md-cluster.c
> >>@@ -10,6 +10,8 @@
> >>  #include <linux/module.h>
> >>+#include <linux/completion.h>
> >>+#include <linux/kthread.h>
> >>  #include <linux/dlm.h>
> >>  #include <linux/sched.h>
> >>  #include <linux/raid/md_p.h>
> >>@@ -141,6 +143,40 @@ static int dlm_unlock_sync(struct dlm_lock_resource *res)
> >>  	return dlm_lock_sync(res, DLM_LOCK_NL);
> >>  }
> >>+/* An variation of dlm_lock_sync, which make lock request could
> >>+ * be interrupted */
> >>+static int dlm_lock_sync_interruptible(struct dlm_lock_resource *res, int mode,
> >>+				       struct mddev *mddev)
> >>+{
> >>+	int ret = 0;
> >>+
> >>+	ret = dlm_lock(res->ls, mode, &res->lksb,
> >>+			res->flags, res->name, strlen(res->name),
> >>+			0, sync_ast, res, res->bast);
> >>+	if (ret)
> >>+		return ret;
> >>+
> >>+	wait_event(res->completion.wait,
> >>+		   res->completion.done || kthread_should_stop());
> >can you convert it to a waitq? Directly using the .wait/.done of completion is
> >really intrusive.
> 
> Maybe, but we still need completion for dlm_lock_resource otherwise there
> are different types of dlm_lock_resource, we also need to keep align with
> sync_ast as dlm_lock_sync did.

Yes, we need a waitq and variable like completion.done to indicate the event is
done, and convert the completion API to waitq API in other places like
sync_ast. The point is not using the opaque data structure of 'struct
completion'. Diving into implementation details of a unrelated data structure
(completion here) is really intrusive.

Thanks,
Shaohua