netdev - Re: [PATCH 4/5] alx: add alx_get

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Date:	Thu, 2 Jan 2014 16:25:06 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Sabrina Dubroca <sd@...asysnail.net>
CC:	David Miller <davem@...emloft.net>, <johannes@...solutions.net>,
	<netdev@...r.kernel.org>
Subject: Re: [PATCH 4/5] alx: add alx_get_stats operation

On Thu, 2014-01-02 at 17:05 +0100, Sabrina Dubroca wrote:
> [2014-01-02, 11:56:53] Ben Hutchings wrote:
> > On Wed, 2014-01-01 at 22:27 -0500, David Miller wrote:
> > > From: Sabrina Dubroca <sd@...asysnail.net>
> > > Date: Thu,  2 Jan 2014 00:40:27 +0100
> > > 
> > > > +	spin_lock(&alx->stats_lock);
> > > > +
> > > > +	__alx_update_hw_stats(&alx->hw);
> > > 
> > > Please use something like the linux/u64_stats_sync.h stuff rather
> > > than spin locking.
> > 
> > I don't think that'sa useful, as we can have multiple writers
> > (concurrent calls to get_stats).  And there is really no harm in using a
> > spinlock to serialise get_stats calls.  The u64_stats API is good for
> > statistics updated from the data path.
> 
> I've read the comments in linux/u64_stats_sync.h, which mentions the
> need for exclusive access to the data. I've looked at other drivers
> (broadcom/b44.c, nvidia/forcedeth.c) that use the u64_stats functions
> to get stats from hardware, and they use a spin_lock around the update
> code. The other drivers that I've looked at and that use u64_stats
> have software stats that they update on rx/tx, so I think the
> situation is a bit different.
> 
> 
> For now I've added u64_stats and modified the functions this way:
[...]

You could do that but I think your original version was fine.  Of course
it is David's decision in the end.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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