Re: [PATCH] arp_queue: serializing unlink + kfree_skb

[Herbert Xu]

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,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
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