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From:   Emil Renner Berthing <kernel@esmil.dk>
Date:   Wed, 27 Jan 2021 17:01:08 +0100
Message-ID: <CANBLGcwfK41+E9JzrU_Hym8VK5S4rGdsyKCHMRiABQvt2zL4kg@mail.gmail.com>
Subject: Re: [PATCH] rtlwifi: use tasklet_setup to initialize rx_work_tasklet
To:     Kalle Valo <kvalo@codeaurora.org>
Cc:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        Ping-Ke Shih <pkshih@realtek.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Allen Pais <allen.lkml@gmail.com>,
        Romain Perier <romain.perier@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, 27 Jan 2021 at 16:33, Kalle Valo <kvalo@codeaurora.org> wrote:
> ...
> Forgot to mention that I can remove the Fixes tags during commit, so no
> need to resend just because of those.

Cool, thanks.

> > I can definitely see how you can reasonably disagree, but I would not
> > be comfortable having code that only works because the calling
> > conventions of all relevant architectures happen to put the first
> > unsigned long argument and the first pointer argument in the same
> > register.
>
> If there's a bug this patch fixes please explain it clearly in the
> commit log. But as I read it (though I admit very quickly) I understood
> this is just cleanup.

Sorry, I'll try again.

So the tasklet_struct looks like this:
struct tasklet_struct {
  ..
  bool use_callback;
  union {
    void (*func)(unsigned long);
    void (*callback)(struct tasklet_struct *);
  };
  unsigned long data;
};

..and the use_callback flag is used like this:
if (t->use_callback)
  t->callback(t);
else
  t->func(t->data);

Now commit d3ccc14dfe95 changed the _rtl_rx_work to be of the new
callback, not func, type. But it didn't actually set the use_callback
flag, and just typecast the _rtl_rx_work function pointer and assigned
it to the func member. So _rtl_rx_work is still called as
t->func(t->data) even though it was rewritten to be called as
t->callback(t).
Now 6b8c7574a5f8 set t->data = (unsigned long)t, so calling
t->func(t->data) will probably work on most architectures because:

a) "unsigned long" and "struct tasklet_struct *" has the same width on
all Linux-capable architectures and
b) calling t->func(t->data) will put the value from t->data into the
same register as the function
    void _rtl_rx_work(struct tasklet_struct *t)
  expects to find the pointer t in the C calling conventions used by
all relevant architectures.

I guess it's debatable weather this is a bug or just ugly code.

/Emil