netdev - RE: [PATCH net-next 1/9] vlan: adopt u64_stats

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Date:   Wed, 8 Jun 2022 10:18:22 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
CC:     netdev <netdev@...r.kernel.org>,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH net-next 1/9] vlan: adopt u64_stats_t

From: Eric Dumazet
> Sent: 08 June 2022 00:36
> 
> As explained in commit 316580b69d0a ("u64_stats: provide u64_stats_t type")
> we should use u64_stats_t and related accessors to avoid load/store tearing.
> 
> Add READ_ONCE() when reading rx_errors & tx_dropped.

Isn't this all getting entirely stupid?

AFAICT nearly every 'memory' access in the kernel is going
to get wrapped in READ/WRITE_ONCE() to avoid something
that really never actually happens?

It might be better to just mark everything 'volatile'.
Although perhaps that ought to be a compiler option.

OTOH I've seen gcc generate extra instructions for 'volatile'
accesses - to the point where I used 'barrier()' to optimise
code.
I think the volatile casts in READ_ONCE() can generate worse
code than volatile variables.

	David

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