On Thu, Feb 03, 2005 at 04:49:22PM -0800, David S. Miller wrote: > > If we see the count dropped to "1", whoever set it to "1" made > sure that all outstanding memory operations (including things > like __skb_unlink()) are globally visible before the > atomic_dec_and_test() which put the thing to "1" from "2". > (and we did use atomic_dec_and_test() since the refcount was > not "1") Example, assuming skb->users is "2": > > cpu 0 cpu 1 > __skb_unlink() > kfree_skb() > kfree_skb() > > If cpu 0 sees the count at "1", it will always see the > __skb_unlink() as well.
This is true if CPU 0 reads the count before reading skb->list. Without a memory barrier, atomic_read and reading skb->list can be reordered. Put it another way, reading skb->list could return a cached value that was read from the main memory prior to the atomic_read. So in order for CPU 0 to always see an up-to-date value of skb->list, it needs to do an smp_rmb() between the atomic_read and reading skb->list. Cheers, -- Visit Openswan at http://www.openswan.org/ Email: Herbert Xu ~{PmV>HI~} <[EMAIL PROTECTED]> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
