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SubjectRe: Lock ordering, inquiring minds want to know.
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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