Re: [PATCH net-next 1/4] net/sched/sch_cake: serialize reconfiguration with the datapath

["Toke Høiland-Jørgensen" <toke@toke.dk>]

> The qdisc is left with autorate disabled but with the estimate installed
> instead of X.  On 32-bit machines the unlocked 64-bit rate_bps store is
> also not atomic, so a reader can observe a torn rate.  iproute2 reaches
> this because parsing 'bandwidth X' sets autorate = 0 and emits both
> TCA_CAKE_BASE_RATE64 and TCA_CAKE_AUTORATE.
>
> Commit the parsed configuration and reconfigure while holding the same
> qdisc lock that serializes the datapath, so the netlink writer and the
> autorate writer can no longer interleave.  Make rate_bps an atomic64_t
> so a 64-bit rate update is atomic on 32-bit machines as well, and keep
> the READ_ONCE()/WRITE_ONCE() annotations for the remaining lockless
> readers (cake_config_dump(), which runs without the lock, and the
> cake_mq shared config, which readers observe under different child
> locks).

So atomic_t.txt has this section:

"SEMANTICS
---------

Non-RMW ops:

The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
smp_store_release() respectively. Therefore, if you find yourself only using
the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
and are doing it wrong."


So AFAICT, we don't need atomic_t, we just need READ/WRITE_ONCE()
annotations?

-Toke