linux-kernel - Re: [PATCH net-next v2 2/2] net: xilinx: axienet: Add statistics support

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Message-ID: <70d817a8-5ee4-4ce2-883a-9e95f15f2855@lunn.ch>
Date: Mon, 12 Aug 2024 23:18:03 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Sean Anderson <sean.anderson@...ux.dev>
Cc: Radhey Shyam Pandey <radhey.shyam.pandey@....com>,
	netdev@...r.kernel.org, Simon Horman <horms@...nel.org>,
	Jakub Kicinski <kuba@...nel.org>,
	Russell King <linux@...linux.org.uk>,
	"David S . Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Paolo Abeni <pabeni@...hat.com>,
	Michal Simek <michal.simek@....com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next v2 2/2] net: xilinx: axienet: Add statistics
 support

On Mon, Aug 12, 2024 at 04:25:16PM -0400, Sean Anderson wrote:
> On 8/12/24 16:22, Andrew Lunn wrote:
> >>  static int __axienet_device_reset(struct axienet_local *lp)
> >>  {
> >>  	u32 value;
> >>  	int ret;
> >>  
> >> +	/* Save statistics counters in case they will be reset */
> >> +	guard(mutex)(&lp->stats_lock);
> >> +	if (lp->features & XAE_FEATURE_STATS)
> >> +		axienet_stats_update(lp, true);
> > 
> > My understanding of guard() is that the mutex is held until the
> > function completes. That is much longer than you need. A
> > scoped_guard() would be better here, and it makes it clear when the
> > mutex will be released.
> 
> We have to hold it until...
> 
> >> +
> >>  	/* Reset Axi DMA. This would reset Axi Ethernet core as well. The reset
> >>  	 * process of Axi DMA takes a while to complete as all pending
> >>  	 * commands/transfers will be flushed or completed during this
> >> @@ -551,6 +595,23 @@ static int __axienet_device_reset(struct axienet_local *lp)
> >>  		return ret;
> >>  	}
> >>  
> >> +	/* Update statistics counters with new values */
> >> +	if (lp->features & XAE_FEATURE_STATS) {
> >> +		enum temac_stat stat;
> >> +
> >> +		write_seqcount_begin(&lp->hw_stats_seqcount);
> >> +		lp->reset_in_progress = false;
> >> +		for (stat = 0; stat < STAT_COUNT; stat++) {
> >> +			u32 counter =
> >> +				axienet_ior(lp, XAE_STATS_OFFSET + stat * 8);
> >> +
> >> +			lp->hw_stat_base[stat] +=
> >> +				lp->hw_last_counter[stat] - counter;
> >> +			lp->hw_last_counter[stat] = counter;
> >> +		}
> >> +		write_seqcount_end(&lp->hw_stats_seqcount);
> 
> ...here
> 
> Which is effectively the whole function. The main reason why I used guard() was to
> simplify the error return cases.

This is why i personally don't like guard. It is not clear you
intended the mutex to be held so long, and that this code actually
requires it. An explicit mutex_unlock() here would make your
intentions clear, or a scoped_guard. I can see guard avoiding some
error path bugs, but i suspect it will introduce other problems when
refactoring code and having to make guesses about what actually needs
the mutex.

	    Andrew