Re: [PATCH 2/2] dmaengine: rz-dmac: fix dead empty check in rz_dmac_chan_get_residue()

[Geert Uytterhoeven]

Hi Maoyi,

On Thu, 21 May 2026 at 16:48, Maoyi Xie <maoyixie.tju@xxxxxxxxx> wrote:
> rz_dmac_chan_get_residue() reads channel->ld_active with
> list_first_entry() and then tests the returned pointer against
> NULL. list_first_entry() never returns NULL. On an empty list it
> returns container_of(&channel->ld_active, struct rz_dmac_desc,
> node), an aliased pointer derived from the list head. The "return
> 0" shortcut is dead code.
>
> If ld_active is ever empty here, current_desc points at
> &channel->ld_active. The subsequent cookie and status processing
> then reads bogus values from the head's neighbouring memory.
>
> ld_active can be empty when a residue query races with descriptor
> completion on another path. The author intent was clear from the
> existing comment on the next-following check, which already
> acknowledges that the descriptor "could now be complete". The
> empty case is the limit of that race.
>
> Use list_first_entry_or_null() so the empty case returns NULL and
> the existing "return 0" path runs.
>
> The same shape has been cleaned up elsewhere, for example in
> commit fbb8bc408027 ("net: qed: Remove redundant NULL checks after list_first_entry()"),
> commit c708d3fad421 ("crypto: atmel - use list_first_entry_or_null to simplify find_dev"),
> and commit 10379171f346 ("ksmbd: use list_first_entry_or_null for opinfo_get_list()").
> This site was missed by those cleanups.
>
> Signed-off-by: Maoyi Xie <maoyixie.tju@xxxxxxxxx>

Thanks for your patch!

> --- a/drivers/dma/sh/rz-dmac.c
> +++ b/drivers/dma/sh/rz-dmac.c
> @@ -723,8 +723,8 @@ static u32 rz_dmac_chan_get_residue(struct rz_dmac_chan *channel,
>         u32 crla, crtb, i;
>
>         /* Get current processing virtual descriptor */
> -       current_desc = list_first_entry(&channel->ld_active,
> -                                       struct rz_dmac_desc, node);
> +       current_desc = list_first_entry_or_null(&channel->ld_active,
> +                                               struct rz_dmac_desc, node);
>         if (!current_desc)
>                 return 0;
>

Note that proposed "[PATCH v5 09/17] dmaengine: sh: rz-dmac: Use
virt-dma APIs for channel descriptor processing" would remove this code.
https://lore.kernel.org/20260512121219.216159-10-claudiu.beznea.uj@xxxxxxxxxxxxxx

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds