Re: flock(fd, LOCK_UN) taking 500ms+ ?

[Matthew Wilcox]

On Wed, Oct 02, 2002 at 10:04:52AM -0700, Andrew Morton wrote:
> sched_yield() sementics changed a lot.  It used to mean "take a quick
> nap", but it now means "go to the back of the runqueue and stay there
> for absolutely ages".  The latter is a closer interpretation of the spec,
> but it has broken stuff which was tuned to the old behaviour.

*nod*.  This code has been around for many many years ;-)

> It's not really clear why that yield is in there at all?  Unless that
> code is really, really slow (milliseconds) then probably it should just
> be deleted.

Heh, you're so focused on perf tuning, Andrew!  It's not a matter of
locking, it's a matter of semantics.  Here's the comment:

 *  FL_FLOCK locks never deadlock, an existing lock is always removed before
 *  upgrading from shared to exclusive (or vice versa). When this happens
 *  any processes blocked by the current lock are woken up and allowed to
 *  run before the new lock is applied.
 *  Andy Walker ([e-mail]), June 09, 1995

> If there really is a solid need to hand the CPU over to some now-runnable
> higher-priority process then a cond_resched() will suffice.

I think that's the right thing to do.  If I understand right, we'll
check needs_resched at syscall exit, so we don't need to do it for
unlocks, right?

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