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From: Marcel Moolenaar <marcel@xcllnt.net>
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Date: Thu, 24 May 2012 15:07:03 -0700
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To: Nathan Whitehorn <nwhitehorn@freebsd.org>
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Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org,
	Marcel Moolenaar <marcel@freebsd.org>, src-committers@freebsd.org
Subject: Re: svn commit: r235931 - head/sys/powerpc/include
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On May 24, 2012, at 2:13 PM, Nathan Whitehorn wrote:

> On 05/24/12 15:45, Marcel Moolenaar wrote:
>> Author: marcel
>> Date: Thu May 24 20:45:44 2012
>> New Revision: 235931
>> URL: http://svn.freebsd.org/changeset/base/235931
>>=20
>> Log:
>>   Fix the memory barriers for CPUs that do not like lwsync and wedge =
or cause
>>   exceptions early enough during boot that the kernel will do ithe =
same.
>>   Use lwsync only when compiling for LP64 and revert to the more =
proven isync
>>   when compiling for ILP32. Note that in the end (i.e. between =
revision 222198
>>   and this change) ILP32 changed from using sync to using isync. As =
per Nathan
>>   the isync is needed to make sure I/O accesses are properly =
serialized with
>>   locks and isync tends to be more effecient than sync.
>>=20
>=20
> This badly breaks the synchronization primitives. The functions =
mb()/wmb()/rmb() need to be sync or lwsync on ILP32, not isync.

I guess I misinterpreted your email in which you stated that isync is =
better.

> Summary:
> 1. *mb() must be lwsync or sync on all machines, except for wmb() =
which could be eieio
> 2. __ATOMIC_ACQ() must be isync (though could be reduced to lwsync =
with bus_space changes)
> 3. __ATOMIC_REL() must be lwsync or sync

This is absolutely not what I concluded from our discussions. I have no =
idea
how we could end up so out of sync...

--=20
Marcel Moolenaar
marcel@xcllnt.net