[dm-devel] [PATCH v2 1/7] libmultipath: fix tur checker locking

[Martin Wilck]

On Fri, 2018-02-09 at 18:17 -0600, Benjamin Marzinski wrote:
> On Sat, Feb 10, 2018 at 12:36:05AM +0100, Martin Wilck wrote:
> > On Sat, 2018-02-10 at 00:28 +0100, Martin Wilck wrote:
> > > Maybe it's easier than we thought. Attached is a patch on top of
> > > yours that I think might work, please have a look. 
> > > 
> > 
> > That one didn't even compile. This one is better.
> > 
> > Martin
> 
> So if we have this ordering
> 
> - checker calls uatomic_xchg() which returns 1 and then gets
> scheduled
> - thread calls uatomic_set() and then runs till it terminates
> - checker calls pthread_cancel()
> 
> You will get Bart's original bug. 

Yes, I realized that overnight :-( I shouldn't post stuff like this
around midnight. But I have another idea in my mind.

Martin

>  I realize that having the condlog()
> after the uatomic_set() in the thread makes this unlikely, but I
> don't
> races like this. I would be happier with simply taking the original
> code
> and moving the condlog(), if neither of my other two options are
> acceptable.
> 
> -Ben
> 
> > 
> > -- 
> > Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
> > SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham
> > Norton
> > HRB 21284 (AG Nürnberg)
> > From afb9c7de3658d49c4f28f6b9ee618a87b806ecdd Mon Sep 17 00:00:00
> > 2001
> > From: Martin Wilck <mwilck at suse.com>
> > Date: Sat, 10 Feb 2018 00:22:17 +0100
> > Subject: [PATCH] tur checker: make sure pthread_cancel isn't called
> > for exited
> >  thread
> > 
> > If we enter the cleanup function as the result of a pthread_cancel
> > by another
> > thread, we don't need to wait for a cancellation any more. If we
> > exit
> > regularly, just tell the other thread not to try to cancel us.
> > ---
> >  libmultipath/checkers/tur.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/libmultipath/checkers/tur.c
> > b/libmultipath/checkers/tur.c
> > index 894ad41c89c3..5d2b36bfa883 100644
> > --- a/libmultipath/checkers/tur.c
> > +++ b/libmultipath/checkers/tur.c
> > @@ -214,15 +214,13 @@ retry:
> >  
> >  static void cleanup_func(void *data)
> >  {
> > -	int running, holders;
> > +	int holders;
> >  	struct tur_checker_context *ct = data;
> >  
> > -	running = uatomic_xchg(&ct->running, 0);
> > +	uatomic_set(&ct->running, 0);
> >  	holders = uatomic_sub_return(&ct->holders, 1);
> >  	if (!holders)
> >  		cleanup_context(ct);
> > -	if (!running)
> > -		pause();
> >  }
> >  
> >  static int tur_running(struct tur_checker_context *ct)
> > @@ -266,6 +264,9 @@ static void *tur_thread(void *ctx)
> >  	pthread_cond_signal(&ct->active);
> >  	pthread_mutex_unlock(&ct->lock);
> >  
> > +	/* Tell main checker thread not to cancel us, as we exit
> > anyway */
> > +	uatomic_set(&ct->running, 0);
> > +
> >  	condlog(3, "%s: tur checker finished, state %s",
> >  		tur_devt(devt, sizeof(devt), ct),
> > checker_state_name(state));
> >  	tur_thread_cleanup_pop(ct);
> > -- 
> > 2.16.1
> > 
> 
> 

-- 
Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)