netdev - Re: [PATCH net] net: dsa: sja1105: Fix sleeping while atomic in .port_hwtstamp

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Message-ID: <CA+h21hqt+iLJ5yCTvnysEoeK08Ce7yRK1+nSniaBd=p=4y0_SQ@mail.gmail.com>
Date:   Tue, 1 Oct 2019 07:59:27 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     Richard Cochran <richardcochran@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: dsa: sja1105: Fix sleeping while atomic in .port_hwtstamp_set

Hi David,

On Tue, 1 Oct 2019 at 03:20, David Miller <davem@...emloft.net> wrote:
>
> From: Vladimir Oltean <olteanv@...il.com>
> Date: Sun, 29 Sep 2019 01:07:45 +0300
>
> > Currently this stack trace can be seen with CONFIG_DEBUG_ATOMIC_SLEEP=y:
>  ...
> > Enabling RX timestamping will logically disturb the fastpath (processing
> > of meta frames). Replace bool hwts_rx_en with a bit that is checked
> > atomically from the fastpath and temporarily unset from the sleepable
> > context during a change of the RX timestamping process (a destructive
> > operation anyways, requires switch reset).
> > If found unset, the fastpath will just drop any received meta frame and
> > not take the meta_lock at all.
> >
> > Fixes: a602afd200f5 ("net: dsa: sja1105: Expose PTP timestamping ioctls to userspace")
> > Signed-off-by: Vladimir Oltean <olteanv@...il.com>
>
> Well, two things:
>
> 1) Even assuming #2 wasn't true, you're adding the missing initialization
>    of meta_lock and that would need to be mentioned in the commit message.
>

Thanks, I forgot to break that line apart into a new patch.

> 2) After these changes meta_lock is no longer used so it should be removed.

The meta_lock is still very much used: see net/dsa/tag_sja1105.c (that
is where DSA keeps its fastpath). The fact that meta_lock is no longer
used within the driver (slowpath) is entirely the point of the patch.

-Vladimir