Re: Lock ordering, inquiring minds want to know.

[george anzinger <george@mvista.com>]

Mike Kravetz wrote:
> 
> George,
> 
> I can't answer your question.  However, have  you noticed that this
> lock ordering has changed in the test11 kernel.  The new sequence is:
> 
>         read_lock_irq(&tasklist_lock);
>         spin_lock(&runqueue_lock);
> 
> Perhaps the person who made this change could provide their reasoning.
> 
> An additional question I have is:  Is it really necessary to hold
> the runqueue lock (with interrupts disabled) for as long as we do
> in this routine (setscheduler())?  I suspect we only need the
> tasklist_lock while calling find_process_by_pid().  Isn't it
> possible to do the error checking (parameter validation) with just
> the tasklist_lock held?  Seems that we would only need to acquire
> the runqueue_lock (and disable interrupts) if we are in fact
> changing the task's scheduling policy.

Yes, I think this is true.  The runqueue_lock should only be needed
after the error checks.  Still, the error checks don't take all that
long...

George
> -
> Mike
> 
> On Thu, Dec 07, 2000 at 03:07:18PM -0800, george anzinger wrote:
> > In looking over sched.c I find:
> >
> >       spin_lock_irq(&runqueue_lock);
> >       read_lock(&tasklist_lock);
> >
> >
> > This seems to me to be the wrong order of things.  The read lock
> > unavailable (some one holds a write lock) for relatively long periods of
> > time, for example, wait holds it in a while loop.  On the other hand the
> > runqueue_lock, being a "irq" lock will always be held for short periods
> > of time.  It would seem better to wait for the runqueue lock while
> > holding the read_lock with the interrupts on than to wait for the
> > read_lock with interrupts off.  As near as I can tell this is the only
> > place in the system that both of these locks are held (of course, all
> > cases of two locks being held at the same time, both locker must use the
> > same order).  So...
> >
> >
> > What am I missing here?
> >
> > George
> -
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