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

[Neil Horman <nhorman@tuxdriver.com>]

On Fri, Sep 26, 2014 at 06:07:14PM +0000, Ananyev, Konstantin wrote:
> 
> 
> > > As I remember the purpose of the patch was to fix the race condition inside rte_alarm library.
> > > I believe that the patch provided by Michal & Pawel fixes the issues you discovered.
> > > If you think, that is not the case, could you please provide a list of remaining issues?
> > > Excluding ones that you just don't like it, and you are not happy with rte_alarm API in total?
> 
> 
> > Gladly.  As Pawel explained the race, its possible that, after calling
> > rte_eal_alarm_cancel, an in-flight execution of an alarm callback may still be
> > running.  The problem with that ostensibly is that data which is being accessed
> > by the callback might be then accessed in parallel with another process leading
> > to data corruption or some other problem. The issue I have with his patch is
> > that it doesn't completely close the race.  While it does close the race for the
> > condition in whcih thread B is running the alarm callback while thread A is
> > executing the cancel operation, it does not close the case for when a single
> > thread B is running the cancel operation, as the in-flight execution itself is
> > still active.
> 
> A bit puzzled here:
> Are you saying that calling alarm_cancel() for itself inside eal_alarm_callback() might cause a problem?
> I still don't see how.
> 
Potentially yes, by the same race condition that exists when using a secondary
thread to do the cancel call.  As I understand it the race that Pawel described
is as follows:

Thread A					Thread B
alarm_cancel()					eal_alarm_callback
    block on alarm spinlock			drop spinlock
        run cancel operation			execute callback function
    return from cancel
    rte_eal_alarm_set				

As Pawel described the problem, there is a desire to not set the new alarm while
the old alarm is still executing.  And his patch accomplishes that for the two
thread case above just fine

The problem with Pawels patch is that its non functional in the case where the
cancel happens within Thread B.  Lets change the scenario just a little bit:

Thread B					Thread C
eal_alarm_callback
     callback_function
          some_other_common_func
               rte_eal_alarm_cancel(this)
          pthread_signal(Thread C)		wake up
          operate on alarm data			rte_eal_alarm_set


In this scenario the problem is not fixed because when called from within the
alarm thread, the executing alarm is skipped (as it must be), but that fact is
invisible to the caller, and because of that its still possible for the same
origional problem to occur.

Neil