Re: [dpdk-dev] [PATCH v2] Change alarm cancel function to thread-safe:

["Wodkowski, PawelX" <pawelx.wodkowski@intel.com>]

> So basically cancel() just set ALARM_CANCELLED and leaves actual alarm
> deletion to the callback()?
> That was the thought, yes.
> 
> > I think it is doable - but I don't see any real advantage with that approach.
> > Yes, code will become a bit simpler, as  we'll have one point when we remove
> alarm from the list.
> Yes, that would be the advantage, that the code would be much simpler.
> 
> > But from other side, imagine such simple test-case:
> >
> > for (i = 0; i < 0x100000; i++) {
> >    rte_eal_alarm_set(ONE_MIN, cb_func, (void *)i);
> >    rte_eal_alarm_cancel(cb_func, (void *)i);
> > }
> >
> > We'll endup with 1M of cancelled, but still not removed entries in the
> alarm_list.
> > With current implementation that means - few MBs of wasted memory,
> Thats correct, and the tradeoff to choose between.  Do you want simpler code
> that is easier to maintain, or do you want a high speed cancel and set
> operation.  I'm not aware of all the use cases, but I have a hard time seeing
> a use case in which the in-flight alarm list grows unboundedly large, which in
> my mind mitigates the risk of deferred removal, but I'm perfectly willing to
> believe that there are use cases which I'm not aware of.
> 
> > plus very slow set() and cancel(), as they'll  have to traverse all entries in the
> list.
> > And all that - for empty from user perspective alarm_list
> > So I still prefer Michal's way.
> > After all, it doesn't look that complicated to me.
> Except that the need for Michals fix arose from the fact that we have two free
> locations that might both get called depending on the situation.  Thats what I'm
> trying to address here, the complexity itself, rather than the fix (which I
> agree is perfectly valid).
> 
> > BTW, any particular reason you are so negative about pthread_self()?
> >
> Nothing specifically against it (save for its inverted return code sense, which
> made it difficult for me to parse when reviewing).  Its more the complexity
> itself in the alarm cancel and callback routine that I was looking at.  Given
> that the origional bug happened because an cancel in a callback might produce a
> double free, I wanted to fix it by simpifying the code, not adding conditions
> which make it more complex.
> 
> You know, looking at it, something else just occured to me.  I think this could
> all be fixed without the cancel flag or the pthread_self assignments.  What if
> we simply removed the alarm from the list before we called the callback in
> rte_eal_alarm_callback()?  That way any cancel operation called from within the
> callback would fail, as it wouldn't appear on the list, and the callback
> operation would be the only freeing entity?  That would let you still have a
> fast set and cancel, and avoid the race.  Thoughts?  Untested sample patch
> below
> 
> 
> > >
> > > It also seems like the alarm api as a whole could use some improvement.
> The
> > > way its written right now, theres no way to refer to a specific alarm (i.e.
> > > cancelation relies on the specification of a function and data pointer, which
> > > may refer to multiple timers).  Shouldn't rte_eal_alarm_set return an opaque
> > > handle to a unique timer instance that can be store by a caller and used to
> > > specfically cancel that timer?  Thats how both the bsd and linux timer
> > > subsystems model timers.
> >
> > Yeh,  alarm API looks a bit unusual.
> > Though, I suppose that's subject for another patch/discussion :)
> >
> Yes, agreed :)
> 

Please read quoted message bellow:

> >
> >
> > His solution *does* eliminate race condition too.
> >
> I applied his patch. And here is the problem
> 1	rte_spinlock_lock(&alarm_list_lk);
> 2	while ((ap = LIST_FIRST(&alarm_list)) !=NULL &&
> 3			gettimeofday(&now, NULL) == 0 &&
> 4			(ap->time.tv_sec < now.tv_sec || (ap->time.tv_sec ==
> now.tv_sec &&
> 5						ap->time.tv_usec <=
> now.tv_usec))){
> 6		ap->executing |= ALARM_EXECUTING;
> 7		if (likely(!(ap->executing & ALARM_CANCELLED))) {
> 8				rte_spinlock_unlock(&alarm_list_lk);
> 9                           //another thread: rte_alarm_cancel called, mark this timer
> canceled and exit ( THE RACE)
> 10				ap->cb_fn(ap->cb_arg); // rte_alarm_set called
> (THE RACE)
> 11
> 12				rte_spinlock_lock(&alarm_list_lk);
> 13		}
> 14
> 15		rte_spinlock_lock(&alarm_list_lk);
> 16		LIST_REMOVE(ap, next);
> 17		rte_free(ap);
> 18	}
> 
> Imagine
> 
> Thread 1:                                         Thread2
> Execute eal_alarm_callback
> Lock list at 1                                   rte_alarm_cancel -> block on spinlock
> ....
> Realease lock at line 8                  rte_alarm_cancel -> resumes execution -> it
> mark this timer canceled
> ap->cb_fn is called at line 10       rte_alarm_cancel exits reporting all alarms are
> canceled and not executing (which is not true!)
>     rte_alarm_set is called
>     to rearm itself (THE RACE)
> 
> He only mark timer to * do not execute* but does not assure that it is not
> executing while canceling.
> Race condition is made by unlocking at line 8 and our solution workarounds this
> by looping until all alarms finish execution then cancel what left after callback
> finish (*including rearmed alarms*).
> Real elimination of the race would be by using recursive locking when *other
> thread can't* access the list *while callback* is running but callback *itself can
> by using recursive locking*.
> 
> Maybe I don't see something obvious? :)
> 
> Pawel