Re: [PATCH 2/2] mac80211: protect skb_queue_len(&ps->bc_buf) by lock

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Wed, Feb 19, 2014 at 03:51:43PM +0100, Johannes Berg wrote:
> On Wed, 2014-02-19 at 14:35 +0100, Stanislaw Gruszka wrote:
> 
> > > It ultimately makes no difference at all, it just makes this code more
> > > difficult to read and understand.
> > 
> > It make difference when queue length value is modified on different CPU
> > and read on different CPU. Without lock you can 'see' old length value
> > on CPU that run ieee80211_tx_h_multicast_ps_buf() for undefined
> > period of time (ok maybe not undefined on x86), and current oldest
> > frame can be not necessarily dropped.
> 
> I don't see how that can be true, since the modifications of the queue
> length are under spinlock with the implied memory barriers.

On processor that read value there is no memory barrier. If you do:

CPU1				CPU2

				b = a;
spin_lock(a_lock)
a = 1;				...
spin_unlock(a_lock)

				b = a;

There is nothing that guarantee that on CPU2 b will be 1.

Stanislaw