From owner-freebsd-current  Tue Aug  8  0:51:15 2000
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Date: Tue, 8 Aug 2000 00:51:00 -0700
From: Alfred Perlstein <bright@wintelcom.net>
To: David Greenman <dg@root.com>
Cc: current@FreeBSD.ORG
Subject: Re: Ugly, slow shutdown
Message-ID: <20000808005100.A4854@fw.wintelcom.net>
References: <200008080238.TAA40871@vashon.polstra.com> <200008080558.WAA05458@implode.root.com>
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* David Greenman <dg@root.com> [000807 23:15] wrote:
> >In article <200008080135.SAA04815@implode.root.com>, David Greenman
> ><dg@root.com> wrote:
> 
> >Obviously the waker-upper knows that the condition is true.  Otherwise
> >the existing code which doesn't check wouldn't work.  In the expensive
> >cases the waker-upper could simply set a flag for the sleeper to
> >check.
> 
>    For me, that doesn't make the code easier to read or understand - it has
> the opposite effect. ...but then I'm used to the historical symantics and
> naturally consider the possible cases when looking at the code.
> 
> >Note, I am not expressing an opinion about whether the sleeps should
> >be terminated prematurely during shutdown.  But I am expressing a
> >strong opinion about whether sleepers should do a reality check before
> >proceeding.
> 
>    I could be persuaded to think that way, but I still remain unconvinced.
> Again, I'm used to the historical symantics, so changing them requires a
> pretty good justification and a bit of brain rewiring, which I naturally
> resist.

It's not just that, if you always have to cover your behind when
doing tsleep you may wind up masking wakeup bugs.  Places like
"vfs_bio.c" line 586 of 3182:

		bp->b_xflags |= BX_BKGRDWAIT;
		tsleep(&bp->b_xflags, PRIBIO, "biord", 0);
		if (bp->b_xflags & BX_BKGRDINPROG)
			panic("bwrite: still writing");
	}

If replaced by a while() _may_ obscure a buffercache bug.

Personally I'd like to be able to catch such bugs than let them go
because the API (wakeups can happen at any time) prohibits this.

-Alfred


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