On Wed, Jul 27, 2016 at 08:15:27AM +0200, Didier Kryn wrote: > Le 26/07/2016 12:59, Rainer Weikusat a écrit : > >Didier Kryn <k...@in2p3.fr> writes: > >>Le 25/07/2016 01:29, Rainer Weikusat a écrit : > >>>Sleeping on a contended mutex is implemented in this way. But that's > >>>supposed to be an exceptional case. > >> This is why, while advertizing itself as a cool "don't care" > >>feature, a mutex is problematic: > >I don't know who's advertising that "don't care" use of shared variables > >would be "cool" and who's being targetted by this advertising (and I > >don't even want to know) but I don't share this opinion. This applies > >equally to single-threaded and multi-threaded applications. A mutex is a > >synchronisation primitive useful for implementing userspace inter-thread > >communication facilities (this is a bit too general). > > > > This bias of mine comes from the fact that, in high level languages, > mutexes are embedded inside dedicated objects, and the application can > assign values to these objects exactly as it would do for any other object > (without specific care). > > I absolutely expected you wouldn't share my opinion :-> > > >>the programmer should make sure contention is exceptional and there is > >>no risk of dead-lock. > >Mutexes (or any other kind of lock) should usually be uncontended if > >performance is considered important. It's often not necessary that > >threads hold more than one mutex at the same time. > > Here we see that we probably think of very different uses cases. > > > In case it is, care > >must be taken to ensure that they always try to acquire all members of > >the shared set of mutexes in the same order to avoid deadlocks. > > > > All threads aquiring locks in the same order, this is equivalent to > having only one lock, no?
No. The requirement isn't that a thread acquire all the locks. It's that there be an ordering of locks so that if a thread acquires locks E, F, G in that order, then E > F > G by the global order. There's no requirement that some other thread can't acquire G without acquiring E or F at all. What it can't do is acquire E or F after acquiring G. -- hendrik _______________________________________________ Dng mailing list Dng@lists.dyne.org https://mailinglists.dyne.org/cgi-bin/mailman/listinfo/dng
