> -----Original Message----- > From: linux-mips-bounce@xxxxxxxxxxxxxx > [mailto:linux-mips-bounce@xxxxxxxxxxxxxx] On Behalf Of Kaz Kylheku > Sent: Monday, November 06, 2006 8:18 PM > To: linux-mips@xxxxxxxxxxxxxx > Subject: RE: Sync operation in atomic_add_return() > > Gideon Stupp wrote: > > Hi, > > I am trying to figure out why there is a sync operation in > > linux/include/asm-mips/atomic.h:atomic_add_return(). > > I believe it was added in the linux-2.4.19 patch, but can't > trace the > > reason. Can anyone help? > > Is it just unwarranted paranoia? There does not appear to be > a need for the sync within the atomic_add_return code itself. > > But it might be that the code which calls this function needs > the sync. > > Without looking at any code whatsoever, here is a general hypothesis. > > In what situation might you /care/ about the return value of > an atomic add? > > Suppose atomic increments and decrements are being used for > reference counting. If you know that you hold the reference > to an object, you can call atomic_add to increase the > reference count without caring about the return value, and no > sync is needed in that situation. > > Suppose, however, that atomic_add is used to pick up a reference. > Suppose you have a pool of ``dead'' objects with reference > counts of zero, and want to recycle an object from such a > pool. You might use atomic_add_return to examine the > reference counts of the objects in this pool one by one until > you get a 1 return. You might get something other than a 1 > return if racing against another processor which is tryiing > to pick up the same object. > > In this situation, if you successfully get the object, you do > want to do a sync, since the object is being handed off > between two processors. > Before the object was put into the pool, its fields were > updated, since it was being cleaned up. You would not want > the new owner, by chance, to observe stale values of those fields. > > I.e., to put it briefly, atomic_add_return can have "acquire" > semantics. > Thanks for the reply. I also checked the Alpha implementation ( the only other architecture I know of with non serializing atomic operations ) and indeed there is an explicit smp_mb() in atomic_add_return() and nowhere else. So I guess this is the convention. Gideon.
