netdev - Re: [PATCH net-next v2 2/3] net: dsa: add Arrow SpeedChips XRS700x driver

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Message-ID: <20201128014505.hl5erlwnatej2io3@skbuf>
Date:   Sat, 28 Nov 2020 03:45:05 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Andrew Lunn <andrew@...n.ch>,
        George McCollister <george.mccollister@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        "open list:OPEN FIRMWARE AND..." <devicetree@...r.kernel.org>
Subject: Re: [PATCH net-next v2 2/3] net: dsa: add Arrow SpeedChips XRS700x
 driver

On Fri, Nov 27, 2020 at 03:56:49PM -0800, Jakub Kicinski wrote:
> What is it with my email today, didn't get this one again.
>
> On Sat, 28 Nov 2020 01:39:16 +0200 Vladimir Oltean wrote:
> > On Sat, Nov 28, 2020 at 12:30:48AM +0100, Andrew Lunn wrote:
> > > > If there is a better alternative I'm all ears but having /proc and
> > > > ifconfig return zeros for error counts while ip link doesn't will lead
> > > > to too much confusion IMO. While delayed update of stats is a fact of
> > > > life for _years_ now (hence it was backed into the ethtool -C API).
> > >
> > > How about dev_seq_start() issues a netdev notifier chain event, asking
> > > devices which care to update their cached rtnl_link_stats64 counters.
> > > They can decide if their cache is too old, and do a blocking read for
> > > new values.
>
> Just to avoid breaking the suggestion that seqfiles don't sleep after
> .start? Hm. I thought BPF slept (or did cond_reshed()) in the middle of
> seq iteration. We should double check that seqfiles really can't sleep.

I don't think that seqfiles must not sleep after start(), at least
that's my interpretation and confirmed by some tests. I added a might_sleep()
in quite a few places and there is no issue now that we don't take the
RCU read-side lock any longer. Have you seen my previous reply to
George:

| > I suppose it doesn't really matter though since the documentation says
| > we can't sleep.
|
| You're talking, I suppose, about these words of wisdom in
| Documentation/filesystems/seq_file.rst?
|
| | However, the seq_file code (by design) will not sleep between the calls
| | to start() and stop(), so holding a lock during that time is a
| | reasonable thing to do. The seq_file code will also avoid taking any
| | other locks while the iterator is active.
|
| It _doesn't_ say that you can't sleep between start() and stop(), right?
| It just says that if you want to keep the seq_file iterator atomic, the
| seq_file code is not sabotaging you by sleeping. But you still could
| sleep if you wanted to.