From owner-cvs-all  Wed Aug 25 23:39:47 1999
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Date: Thu, 26 Aug 1999 01:39:21 -0500
From: Alan Cox <alc@cs.rice.edu>
To: Stephen McKay <syssgm@detir.qld.gov.au>
Cc: cvs-committers@freebsd.org, cvs-all@freebsd.org
Subject: Re: cvs commit: src/sys/vm vm_map.h
Message-ID: <19990826013921.D27804@cs.rice.edu>
References: <199908231808.LAA86314@freefall.freebsd.org> <199908250703.RAA14106@nymph.detir.qld.gov.au>
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In-Reply-To: <199908250703.RAA14106@nymph.detir.qld.gov.au>; from Stephen McKay on Wed, Aug 25, 1999 at 05:03:24PM +1000
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On Wed, Aug 25, 1999 at 05:03:24PM +1000, Stephen McKay wrote:
> On Monday, 23rd August 1999, Alan Cox wrote:
> 
> >alc         1999/08/23 11:08:35 PDT
> >
> >  Modified files:
> >    sys/vm               vm_map.h 
> >  Log:
> >  struct vm_map:
> >  	The lock structure cannot be the first element of the vm_map
> >  	because this can result in livelock between two or more system
> >  	processes trying to kmem_alloc_wait.
> 
> Wouldn't it have been better to change the lock code to sleep on a field
> within the lock structure, rather than the first byte?
> 

Yes and no.  :-)  Doing as you suggest is a great idea because it could
reduce the chance of this same problem happening elsewhere in the kernel.
(So, I would welcome a patch that implements it.)

On the other hand, this patch fixes the problem at hand and has a near-zero 
chance of causing any new problems.  I don't feel the same way about any
changes to lockmgr, and since 3.x-STABLE is subject to the same problem,
I wanted to try the simplest patch, that I wouldn't worry about MFC'ing,
first.

Alan


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