cvs commit: src/sys/kern kern_proc.c

Pawel Jakub Dawidek pjd at
Wed Jun 9 18:47:32 GMT 2004

On Wed, Jun 09, 2004 at 05:53:57PM +0000, Bosko Milekic wrote:
+> Pawel Jakub Dawidek wrote:
+> >But isn't you reference counting mechanism limited to only 0 and 1
+> >values?
+>   Nope.
+>   The cmpset does exactly this, atomically: if the refcnt is 0, set it
+>   to 1 and return non-zero, otherwise leave it as it and return 0.
+>   Therefore, if two threads are freeing at the same time, the refcount
+>   will get dropped twice [atomically, so we don't have to worry about
+>   a missed decrement], and the threads will race on that atomic cmpset.
+>   But since the cmpset is atomic, then only one thread will get to set
+>   the refcnt to 1 and free, and the other will see that it is not zero,
+>   and so its cmpset will merely return 0 and it will be done (it won't
+>   have to be the one cleaning up/freeing the object).
+>   The reference count, after hitting zero, cannot go back up because the
+>   object is being freed and no other references exist.  If they do, then
+>   the reference counting model is broken.
+>   Note that in the cmpset, if the refcnt is NOT zero, all that has happened
+>   is that it was decremented by 1 and the object not freed.
+>   Again, the code is correct.

Ok, I get it now. Even if there will be a race between two threads, i.e.:

	[ref_cnt is 2]

	thread1					thread2

	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]

(thread2 sets ref_cnt to 0, but thread1 destroys objects) we free object
only once. Cool, I like it:)

BTW. Do we assume that atomic operations are atomic on MP machines?
I read atomic(9), but I still don't know if I should use it or not for
things like those.

Pawel Jakub Dawidek             
pjd at                 
FreeBSD committer                         Am I Evil? Yes, I Am!
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