Lock ordering, inquiring minds want to know.

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

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.

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Mike

Quoted 18 lines · 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...

Quoted 3 lines

What am I missing here?

George

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