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Date: Tue, 31 May 2005 20:19:08 +0000
From: Alexander Kabaev <kan@freebsd.org>
To: Maksim Yevmenkin <maksim.yevmenkin@savvis.net>
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Cc: Norbert Koch <NKoch@demig.de>, freebsd-hackers@freebsd.org
Subject: Re: mutual exclusion in vkbd
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On Tue, May 31, 2005 at 09:41:18AM -0700, Maksim Yevmenkin wrote:
> Norbert,
> 
> >I am currently trying to backport vkbd to FreeBSD 4.
> 
> ok
> 
> >Maksim Yevmenkin uses mtx_lock()/mtx_unlock() for
> >protecting access to data structures under FreeBSD 5/6
> >between the device functions and the kernel thread.
> >
> >How should I best do this under FreeBSD 4?
> >
> >Would something like splhigh() work in that context?
> >Or should I use lockmgr with LK_EXCLUSIVE/LK_RELEASE?
> >Is there any (pseudo)process context inside a kernel task?
> 
> spltty() is what you probably need to use. you could just adjust the 
> following defines like
> 
> #define VKBD_LOCK_DECL		int
> #define VKBD_LOCK_INIT(s)	/* noop */
> #define VKBD_LOCK_DESTROY(s)	/* noop */	
> #define VKBD_LOCK(s)		(s)->ks_lock = spltty()
> #define VKBD_UNLOCK(s)		splx((s)->ks_lock)
> #define VKBD_LOCK_ASSERT(s, w)
> #define VKBD_SLEEP(s, f, d, t) \
> 	tsleep(&(s)->f, PCATCH | (PZERO + 1), d, t)
> 

The code above will probably crash the kernel in many spectacular and
unpredictable ways. You will need to save interrupt flags locally to each
VKBD_LOCK caller or they will end up restoring each other's flags.

--
Alexander Kabaev