[PATCH RFC net-next 03/14] net: phylink: add support for PCS link change notifications

[Sean Anderson]

On 1/23/24 15:07, Russell King (Oracle) wrote:
> On Tue, Jan 23, 2024 at 02:46:15PM -0500, Sean Anderson wrote:
>> Hi Russell,
>> 
>> Does there need to be any locking when calling phylink_pcs_change? I
>> noticed that you call it from threaded IRQ context in [1]. Can that race
>> with phylink_major_config?
> 
> What kind of scenario are you thinking may require locking?

Can't we at least get a spurious bounce? E.g.

pcs_major_config()
  pcs_disable(old_pcs) /* masks IRQ */
  old_pcs->phylink = NULL;
  new_pcs->phylink = pl;
  ...
  pcs_enable(new_pcs) /* unmasks IRQ */
  ...

pcs_handle_irq(new_pcs) /* Link up IRQ */
  phylink_pcs_change(new_pcs, true)
    phylink_run_resolve(pl)

phylink_resolve(pl)
  /* Link up */

pcs_handle_irq(old_pcs) /* Link down IRQ (pending from before pcs_disable) */
  phylink_pcs_change(old_pcs, false)
    phylink_run_resolve(pl) /* Doesn't see the NULL */

phylink_resolve(pl)
  /* Link down; retrigger */
phylink_resolve(pl)
  /* Link up */

And mac_link_dropped probably needs WRITE_ONCE in order to take
advantage of the ordering provided by queue_work.

> I guess the possibility would be if pcs->phylink changes and the
> compiler reads it multiple times - READ_ONCE() should solve that.
> 
> However, in terms of the mechanics, there's no race.
> 
> During the initial bringup, the resolve worker isn't started until
> after phylink_major_config() has completed (it's started at
> phylink_enable_and_run_resolve().) So, if phylink_pcs_change()
> gets called while in phylink_major_config() there, it'll see
> that pl->phylink_disable_state is non-zero, and won't queue the
> work.
> 
> The next one is within the worker itself - and there can only
> be one instance of the worker running in totality. So, if
> phylink_pcs_change() gets called while phylink_major_config() is
> running from this path, the only thing it'll do is re-schedule
> the resolve worker to run another iteration which is harmless
> (whether or not the PCS is still current.)
> 
> The last case is phylink_ethtool_ksettings_set(). This runs under
> the state_mutex, which locks out the resolve worker (since it also
> takes that mutex).
> 
> So calling phylink_pcs_change() should be pretty harmless _unless_
> the compiler re-reads pcs->phylink multiple times inside
> phylink_pcs_change(), which I suppose with modern compilers is
> possible. Hence my suggestion above about READ_ONCE() for that.
> 
> Have you encountered an OOPS because pcs->phylink has become NULL?
> Or have you spotted another issue?

I was looking at extending this code, and I was wondering if I needed
to e.g. take RTNL first. Thanks for the quick response.

--Sean