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Date: Wed, 9 Jun 2004 21:02:58 +0200
From: Pawel Jakub Dawidek <pjd@FreeBSD.org>
To: Julian Elischer <julian@elischer.org>
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Subject: Re: cvs commit: src/sys/kern kern_proc.c
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On Wed, Jun 09, 2004 at 11:54:06AM -0700, Julian Elischer wrote:
+> > Ok, I get it now. Even if there will be a race between two threads, i.=
e.:
+> >=20
+> > 	[ref_cnt is 2]
+> >=20
+> > 	thread1					thread2
+> >=20
+> > 	atomic_substract(obj, 1)
+> > 	[ref_cnt is 1]
+> > 						atomic_substract(obj, 1)
+> > 						[ref_cnt is 0]
+> > 	atomic_cmpset(obj, 0, 1) -> true
+> > 	[do clean up]
+> > 						atomic_cmpset(obj, 0, 1) -> false
+> > 						[skip cleanups]
+> >=20
+> > (thread2 sets ref_cnt to 0, but thread1 destroys objects) we free obje=
ct
+> > only once. Cool, I like it:)
+>=20
+> The method shown above has, however the disadvantage of being
+> less than obvious.  ALso thread 2 is likely to get a page fault, which
+> is a less than perfect result..
+>=20
+> the reference count must never be read or written after the [cleanup]
+> because  it may free the page on which teh counter is kept.

Right... Nice catch. So Bosko probebly have to rethink his model.

--=20
Pawel Jakub Dawidek                       http://www.FreeBSD.org
pjd@FreeBSD.org                           http://garage.freebsd.pl
FreeBSD committer                         Am I Evil? Yes, I Am!

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