Lock ordering, inquiring minds want to know.

Lock ordering, inquiring minds want to know.

[george anzinger]

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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Re: Lock ordering, inquiring minds want to know.

[Mike Kravetz]

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.
-
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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Re: Lock ordering, inquiring minds want to know.

[george anzinger]

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
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> Please read the FAQ at http://www.tux.org/lkml/
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