Message-ID: <20240905135440.hcgbva7fzic2x4ps@skbuf>
Date: Thu, 5 Sep 2024 16:54:40 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Furong Xu <0x1207@...il.com>
Cc: Alexander Lobakin <aleksander.lobakin@...el.com>,
	Serge Semin <fancer.lancer@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Jose Abreu <joabreu@...opsys.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Joao Pinto <jpinto@...opsys.com>, netdev@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	rmk+kernel@...linux.org.uk, linux@...linux.org.uk, xfr@...look.com,
	Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next v8 3/7] net: stmmac: refactor FPE verification
 process

On Thu, Sep 05, 2024 at 03:02:24PM +0800, Furong Xu wrote:
> +void stmmac_fpe_apply(struct stmmac_priv *priv)
> +{
> +	struct ethtool_mm_state *state = &priv->fpe_cfg.state;
> +	struct stmmac_fpe_cfg *fpe_cfg = &priv->fpe_cfg;
> +
> +	/* If verification is disabled, configure FPE right away.
> +	 * Otherwise let the timer code do it.
> +	 */
> +	if (!state->verify_enabled) {
> +		stmmac_fpe_configure(priv, priv->ioaddr, fpe_cfg,
> +				     priv->plat->tx_queues_to_use,
> +				     priv->plat->rx_queues_to_use,
> +				     state->tx_enabled,
> +				     state->pmac_enabled);
> +	} else {
> +		state->verify_status = ETHTOOL_MM_VERIFY_STATUS_INITIAL;
> +		fpe_cfg->verify_retries = STMMAC_FPE_MM_MAX_VERIFY_RETRIES;
> +
> +		if (netif_device_present(priv->dev) && netif_running(priv->dev))
> +			stmmac_fpe_verify_timer_arm(fpe_cfg);
> +	}
>  }

In the cover letter, you say:

  2. check netif_running() to guarantee synchronization rules between
  mod_timer() and timer_delete_sync()

[ by the way, it would be nice if you could list the changes in
  individual patches as well ]

but I guess this helps with something other than what you say it helps
with.

netif_running() essentially checks that __dev_open() has been called,
aka "ip link set dev eth0 up". And I don't see the ethtool_ops :: begin()
implemented by the driver any longer, so I think you've done this in
order to accept stmmac_set_mm() calls even before the netdev has been
brought operationally up. Okay.

As for netif_device_present(), I don't know, maybe the intention was to
suppress stmmac_set_mm() calls made after stmmac_suspend(). But
ethnl_ops_begin() has its own netif_device_present() call, so I'm not
sure why it is needed - they should already be suppressed.

But in v7, I was thinking about the concurrency issues here:

static int stmmac_set_mm(struct net_device *ndev, struct ethtool_mm_cfg *cfg,
			 struct netlink_ext_ack *extack)
{
	/* Wait for the verification that's currently in progress to finish */
	del_timer_sync(&fpe_cfg->verify_timer);

								<- Concurrent code can run here:
								   stmmac_fpe_link_state_handle(),
								   called from phylink_resolve()
								   workqueue context, rtnl_lock()
								   not held.

	spin_lock_irqsave(&fpe_cfg->lock, flags);
	stmmac_fpe_apply(priv);
	spin_unlock_irqrestore(&fpe_cfg->lock, flags);
}

static void stmmac_fpe_link_state_handle(struct stmmac_priv *priv, bool is_up)
{
	struct stmmac_fpe_cfg *fpe_cfg = &priv->fpe_cfg;
	unsigned long flags;

	timer_delete_sync(&fpe_cfg->verify_timer);

								<- Concurrent code can run here:
								   stmmac_set_mm()

	spin_lock_irqsave(&fpe_cfg->lock, flags);

	if (is_up && fpe_cfg->state.pmac_enabled) {
		/* VERIFY process requires pmac enabled when NIC comes up */
		stmmac_fpe_configure(priv, priv->ioaddr, fpe_cfg,
				     priv->plat->tx_queues_to_use,
				     priv->plat->rx_queues_to_use,
				     false, true);

		/* New link => maybe new partner => new verification process */
		stmmac_fpe_apply(priv);
	} else {
		/* No link => turn off EFPE */
		stmmac_fpe_configure(priv, priv->ioaddr, fpe_cfg,
				     priv->plat->tx_queues_to_use,
				     priv->plat->rx_queues_to_use,
				     false, false);
	}

	spin_unlock_irqrestore(&fpe_cfg->lock, flags);
}

[ oh btw, you forgot to replace the del_timer_sync() instance from
  stmmac_set_mm() to timer_delete_sync() ]

Because the timer can be restarted right after the timer_delete_sync()
call, this is a half-baked implementation.

I think at the end of the day, we need to ask ourselves: what is the
timer_delete_sync() call even supposed to accomplish? What if the verify
timer is allowed to run concurrently with us changing the settings?

Well, for example, if it runs concurrently with
stmmac_fpe_link_state_handle(is_down==false), it will not learn that the
link is down, it will send an MPACKET_VERIFY, get no response, and fail.
So, not very bad.

And the other way around: stmmac_set_mm() stops the verify timer, but
the link comes up, the timer is armed with the old settings, it does
whatever (succeeds, fails), and only afterwards does stmmac_set_mm()
manage to grab &fpe_cfg->lock, change the settings to the new ones, and
re-trigger the verify timer once again, if needed.

So bottom line, I think timer_delete_sync() is to avoid some useless
work, but otherwise, it is not critical to have it. The choice is
between removing the timer_delete_sync() calls from these 2 functions
altogether, or implementing an actually effective mechanism to stop the
timer for a while.

I _think_ that the simplest way to stop it is to hold one more lock for
the verify_timer when we call timer_delete_sync() and stmmac_fpe_verify_timer_arm(),
lock which _is_ IRQ-safe, unlike &fpe_cfg->lock.

static int stmmac_set_mm(struct net_device *ndev, struct ethtool_mm_cfg *cfg,
			 struct netlink_ext_ack *extack)
{
	spin_lock(&fpe_cfg->verify_timer_lock);

	timer_delete_sync(&fpe_cfg->verify_timer);

	spin_lock_irqsave(&fpe_cfg->lock, flags);
	stmmac_fpe_apply(priv);
	spin_unlock_irqrestore(&fpe_cfg->lock, flags);

	spin_unlock(&fpe_cfg->verify_timer_lock);
}

static void stmmac_fpe_link_state_handle(struct stmmac_priv *priv, bool is_up)
{
	spin_lock(&fpe_cfg->verify_timer_lock);

	timer_delete_sync(&fpe_cfg->verify_timer);

	spin_lock_irqsave(&fpe_cfg->lock, flags);

	if (is_up && fpe_cfg->state.pmac_enabled) {
		/* VERIFY process requires pmac enabled when NIC comes up */
		stmmac_fpe_configure(priv, priv->ioaddr, fpe_cfg,
				     priv->plat->tx_queues_to_use,
				     priv->plat->rx_queues_to_use,
				     false, true);

		/* New link => maybe new partner => new verification process */
		stmmac_fpe_apply(priv);
	} else {
		/* No link => turn off EFPE */
		stmmac_fpe_configure(priv, priv->ioaddr, fpe_cfg,
				     priv->plat->tx_queues_to_use,
				     priv->plat->rx_queues_to_use,
				     false, false);
	}

	spin_unlock_irqrestore(&fpe_cfg->lock, flags);

	spin_unlock(&fpe_cfg->verify_timer_lock);
}

Looking at the __timer_delete_sync() implementation, I don't think
verify_timer_lock needs to be sleepable and hence a mutex (except on
PREEMPT_RT where spinlocks are sleepable no matter what you do).

But I think the implementation would be simpler without
timer_delete_sync() in these 2 functions, and this overengineered
mechanism.


I would expect a comment in stmmac_release() here:

	if (priv->dma_cap.fpesel)
		timer_delete_sync(&priv->fpe_cfg.verify_timer);

that timer restarts are not possible, because we have rtnl_lock() held
and a concurrent stmmac_set_mm() cannot run now, and the earlier
phylink_stop() has also ensured stmmac_fpe_link_state_handle() cannot
run any longer.

Similarly, I would like to see an explanation in the form of a comment
for why timer restarts are not possible after the same pattern in
stmmac_suspend(). The explanation is different there, I think.

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