Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation

Paul E. McKenney
2010-04-18T13:55:06Z

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.

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.

Thanx, Paul