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From: Bruce Evans <bde@zeta.org.au>
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To: Don Lewis <truckman@FreeBSD.org>
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cc: current@FreeBSD.org
Subject: Re: CFR: fifo_open()/fifo_close() patch
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On Sat, 17 May 2003, Don Lewis wrote:

> On 17 May, Bruce Evans wrote:
> > My question is mainly: why do you want or need the extra complexity for
> > using the vnode interlock here?
>
> I want to use the vnode interlock so that I can msleep() on it to avoid
> a race condition.  If I rely on the vnode lock to protect fi_readers and
> fi_writers, another thread could sneak in, update them, and call
> wakeup() between the VOP_UNLOCK() call and the tsleep() call, causing
> the thread calling tsleep() to miss the wakeup().

I see.

I now think fifo_close() needs both the vnode lock and the interlock.
Its socantrcvmore() calls should be atomic with decrementing the
reader/writer counts to 0.  I think locking them with the interlock
would work, but this depends too much on their internals (not sleeping).
Sorry, I deleted your original patch and don't remember exactly what it
does here.

NetBSD changed VOP_CLOSE() to "L L L" 4+ years ago.

Bruce