On Sun, 2010-04-18 at 06:55 -0700, Paul E. McKenney wrote: > On Sat, Apr 17, 2010 at 10:06:36PM -0500, James Bottomley wrote: > > On Fri, 2010-04-16 at 09:45 -0700, Paul E. McKenney wrote: > > > o mutex_lock(): Critical sections need not guarantee > > > forward progress, as general blocking is permitted. > > > > This isn't quite right. mutex critical sections must guarantee eventual > > forward progress against the class of other potential acquirers of the > > mutex otherwise the system will become either deadlocked or livelocked. > > If I understand you correctly, you are saying that it is OK for a given > critical section for a given mutex to fail to make forward progress if > nothing else happens to acquire that mutex during that time. I would > agree, at least I would if you were to further add that the soft-lockup > checks permit an additional 120 seconds of failure to make forward progress > even if something -is- attempting to acquire that mutex. Yes ... I was thinking of two specific cases: one is wrong programming of lock acquisition where the system deadlocks; the other is doing silly things like taking a mutex around an event loop instead of inside it so incoming events prevent forward progress and the system livelocks, but there are many other ways of producing deadlocks and livelocks. I just couldn't think of a concise way of saying all of that but I didn't want a statement about mutexes to give the impression that anything goes. I've got to say that I also dislike seeing any form of sleep within a critical section because it's just asking for a nasty entangled deadlock which can be very hard to sort out. So I also didn't like the statement "general blocking is permitted" > By my standards, 120 seconds is a reasonable approximation to infinity, > hence my statement above. > > So, would you agree with the following as a more precise statement? > > o mutex_lock(): Critical sections need not guarantee > forward progress unless some other task is waiting > on the mutex in question, in which case critical sections > should complete in 120 seconds. Sounds fair. James -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
