RE: [PATCH net-next v2 2/3] bnxt: use READ_ONCE/WRITE_ONCE for ring indexes

[David Laight <David.Laight@ACULAB.COM>]

From: Jakub Kicinski
> Sent: 12 April 2023 02:51
> 
> Eric points out that we should make sure that ring index updates
> are wrapped in the appropriate READ_ONCE/WRITE_ONCE macros.
> 
...
> -static inline u32 bnxt_tx_avail(struct bnxt *bp, struct bnxt_tx_ring_info *txr)
> +static inline u32 bnxt_tx_avail(struct bnxt *bp,
> +				const struct bnxt_tx_ring_info *txr)
>  {
> -	/* Tell compiler to fetch tx indices from memory. */
> -	barrier();
> +	u32 used = READ_ONCE(txr->tx_prod) - READ_ONCE(txr->tx_cons);
> 
> -	return bp->tx_ring_size -
> -		((txr->tx_prod - txr->tx_cons) & bp->tx_ring_mask);
> +	return bp->tx_ring_size - (used & bp->tx_ring_mask);
>  }

Doesn't that function only make sense if only one of
the ring index can be changing?
In this case I think this is being used in the transmit path
so that 'tx_prod' is constant and is either already read
or need not be read again.

Having written a lot of 'ring access' functions over the years
if the ring size is a power of 2 I'd mask the 'tx_prod' value
when it is being used rather than on the increment.
(So the value just wraps modulo 2**32.)
This tends to make the code safer - especially since the
'ring full' and 'ring empty' conditions are different.

Also that code is masking with bp->tx_ring_mask, but the
increments (in hunks I've chopped) use NEXT_TX(prod).
If that is masking with bp->tx_ring_mask then 'bp' should
be a parameter.

	David

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