From owner-cvs-all  Fri Jan 12 10:37:54 2001
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From: =?ISO-8859-1?Q?G=E9rard_Roudier?= <groudier@club-internet.fr>
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To: John Baldwin <jhb@FreeBSD.org>
Cc: Bruce Evans <bde@zeta.org.au>, cvs-all@FreeBSD.org,
	cvs-committers@FreeBSD.org, Jake Burkholder <jake@FreeBSD.org>
Subject: Re: cvs commit: src/sys/alpha/alpha interrupt.c machdep.c mp_mac
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On Thu, 11 Jan 2001, John Baldwin wrote:

> When the kernel becomes pre-emptive (a pre-emptive kernel is already stab=
le on
> UP) the locking rules for most per-cpu variables will change (with the
> exception of curproc, curpcb, and things tied to the current thread of
> execution).  You will no longer be able to cache and use per-cpu variable=
s
> unless you hold a spin lock to prevent yourself from being pre-empted, si=
nce
> otherwise there is nothing to prevent an interrupt coming in and pre-empt=
ing
> your CPU and you end up running on some other CPU later.  We want people =
to know
> that they are messing with a per-cpu variable that needs special treatmen=
t.

I missed the effect of migrating a thread/process to another CPU. Thanks
for fixing me. Indeed programmer must be aware about coherency when
accessing these variables.

  G=E9rard.

PS: Using ugly macros may well discourage from accessing these variables
when it is not actually needed, and, in some way, it can be considered as
a good point. :-)



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