cvs commit: src/sys/kern kern_umtx.c src/sys/sys proc.h

[Mike Makonnen]

On Thu, Jul 17, 2003 at 10:57:07AM -0400, Mike Makonnen wrote:
> On Thu, Jul 17, 2003 at 09:12:30AM -0400, John Baldwin wrote:
> > >   ...
> > >   2. Once a umtx was contested all future locks and unlocks
> > >      were happening in the kernel, regardless of whether it
> > >      was contested or not. To prevent this from happening,
> > >      when a thread locks a umtx it checks the queue for that
> > >      umtx and unsets the contested bit if there are no other
> > >      threads waiting on it. Again, this is slightly more
> > >      complicated than it needs to be because we can't hold
> > >      a lock across casuptr(). So, the thread has to check
> > >      the queue again after unseting the bit, and reset the
> > >      contested bit if it finds that another thread has put
> > >      itself on the queue in the mean time.
> > >   ...
> > 
> > The in-kernel mutexes handle this case during the unlock
> > routine.  When unlocking a contested mutex, if there is only
> > one waiter, we wake up that waiter and set the lock cookie
> > to MTX_UNOWNED without the contested bit set.  If the woken
> > thread is the only thread to try to acquire the lock, it
> > suceeds and the lock remains uncontested until another thread
> > blocks trying to lock it.  This would mean that the next lock of
> > the lock would not need to bounce into the kernel if the lock
> > was not owned by anyone.
> 
> Initially I was going to do it this way. But there were a couple of
> things that concerned me so I thought it would be safer to implement
> it the way I did. One of them is that it opens up a race between a
> thread that is trying to acquire the umtx for the first time, and the
> thread that the unlocker just woke up. I suppose we could introduce
> a UMTX_RESERVED or something (UMTX_UNOWNED is already used) that no
> one except a thread that just woke up could lock. Is that what you
> are suggesting?
> 

In fact, now that I think about it, this race exists even now. So, I
think your suggestion is needed regardless of where the unlock happens.

Cheers.
-- 
Mike Makonnen  | GPG-KEY: http://www.identd.net/~mtm/mtm.asc
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