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Date: Thu, 5 Jul 2001 19:14:47 -0700 (PDT)
From: Matt Dillon <dillon@earth.backplane.com>
Message-Id: <200107060214.f662ElT61708@earth.backplane.com>
To: Alfred Perlstein <bright@sneakerz.org>
Cc: "Justin T. Gibbs" <gibbs@scsiguy.com>,
	John Baldwin <jhb@FreeBSD.org>, Jake Burkholder <jake@FreeBSD.org>,
	cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org,
	Matthew Jacob <mjacob@feral.com>, Doug Rabson <dfr@nlsystems.com>
Subject: Re: cvs commit: src/sys/sys systm.h condvar.h src/sys/kern kern_
References: <XFMail.010705123747.jhb@FreeBSD.org> <200107052228.f65MSeU64741@aslan.scsiguy.com> <20010705174135.A79818@sneakerz.org>
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:* Justin T. Gibbs <gibbs@scsiguy.com> [010705 17:28] wrote:
:> >It happens with SMP, too, not just preemption.  The calls are an optimization
:> >to avoid problems with releasing the lock after the wakeup.  The contention
:> >can be avoided if we release the lock before calling wakeup(), but doing that
:> >leaves a window open for another CPU to alter the data that the lock protects
:> >possibly invalidating the wakeup that then gets sent.
:> 
:> This window exists anyway.  The locked mutex it not passed to the woken
:> up thread, so there will always be a race between the woken up thread
:> acquiring the mutex and some other thread on some other CPU acquiring it
:> first and making the wakeup invalid.
:
:
:Y'know this sorta got me thinking about something else, shouldn't the
:wakeup() calls for most exclusive locks use wakeup_one?  I know
:wakeup_one() hoses priority, but for the locks in things like vnodes
:and the pager locks, shouldn't we do a wakeup_one() since it is an
:exclusive lock?
:
:-- 
:-Alfred Perlstein [alfred@freebsd.org]

    I would not recommend using wakeup_one until 5.2ish.  A mistake could
    result in hard-to-find system lockups.

						-Matt
    

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