[dm-devel] [PATCH V3] multipathd: release uxsocket and resource when cancel thread

[Wuchongyun]

On Tue, Jan 17, 2018 at 06:39:20PM +0100, Benjamin Marzinski wrote:
> On Tue, Jan 16, 2018 at 02:19:20PM +0100, Martin Wilck wrote:
> > On Tue, 2018-01-16 at 11:48 +0000, Wuchongyun wrote:
> > > Hi Martin,
> > > Sorry to forget that, actually I found that dead_client() will not 
> > > be interrupt by thread cancle, because after all dead_client() 
> > > calling point be done then handle_signals() have chance to be called 
> > > by
> > > uxsock_listen() which will call exit_daemon() and send cancel 
> > > threads signal to all child process include uxlsnr.
> > 
> > Fair enough.
> > 
> > > But your comments is good can make code more safer. Below is the new 
> > > patch, please have a look, thanks.
> > 
> > I think it's really safer whis way, should anyone see the need to 
>>  cancel the listener thread from another point in the code.

> I'm confused why this is safe. After uxsock_listen() calls exit_daemon() from handle_signals(), it doesn't exit. It loops around and polls again, and could in theory find a client that has died.  In fact if the client is killing multipathd via
> # multipathd shutdown
> instead of a signal, won't it be very likely that it will find a dead client when it loops right after calling exit_daemon() in cli_shutdown()? This could hit the deadlock that you noticed, where
> uxsock_cleanup() can't run because dead_client() already holding the mutex.
> Or am I missing something here?

Hi Benjiamin,
Thanks for your comments below are my rely, thanks.

You really found the scenario which need to add pthread_cleanup_push(cleanup_lock, &client_lock) before get lock in dead_client to avoid the dead lock:
If the client is killing multipathd via multipathd shutdown and it find a dead client when it loops right after calling exit_daemon() in cli_shutdown(), This will not hit deadlock because in dead_client before get lock we call pthread_cleanup_push(cleanup_lock, &client_lock) first, then thread cancelation happened, thread cleanup functions been pop up by reverse order: cleanup_lock() first, then uxsock_cleanup(), which make sure client_lock been release before calling uxsock_cleanup(). So it's safer in this way.

And your next comment is right I will make another patch for this.
>Since you are now closing ux_sock in uxsock_cleanup(), you should remove the close(ux_sock) at the end of uxsock_listen(). pthread_cleanup_pop() will already take care of that.

Regards
Chongyun