linux-kernel - Re: [PATCH 1/2] net: ethernet: sxgbe: remove private tx queue lock

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Message-ID: <20161215210324.GA13878@amd>
Date:   Thu, 15 Dec 2016 22:03:24 +0100
From:   Pavel Machek <pavel@....cz>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     Francois Romieu <romieu@...zoreil.com>, bh74.an@...sung.com,
        ks.giri@...sung.com, vipul.pandya@...sung.com,
        peppe.cavallaro@...com, alexandre.torgue@...com,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] net: ethernet: sxgbe: remove private tx queue lock

Hi!

> sorry for the late reply.

No problem. Thanks for the help.

> On 11.12.2016 21:11, Pavel Machek wrote:
> > 
> > Do you understand what stmmac_tx_err(priv); is supposed to do? In
> > particular, if it is called while the driver is working ok -- should
> > the driver survive that?
> 
> As far as I understood it is supposed to fixup an errorneous tx path, e.g. a
> missing tx completion for transmitted frames.
> 
> Some drivers do this by restarting only the HW parts responsible for tx, some
> others by restarting the complete hardware. 
> But IMO it should also be ok to be called if the HW is still working fine.
> 
> > Because it does not currently, and I don't know how to test that
> > code. Unplugging the cable does not provoke that.
> > 
> > I tried
> > 
> >         } else if (unlikely(status == tx_hard_error))
> >                 stmmac_tx_err(priv);
> > +
> > +       {
> > +               static int i;
> > +               i++;
> > +               if (i==1000) {
> > +                       i = 0;
> > +                       printk("Simulated error\n");
> > +                       stmmac_tx_err(priv);
> > +               }
> > +       }
> >  }
> > 
> 
> Ok, there is this race that Francois mentioned so it is not surprising that
> the driver does not survive the call of stmmac_tx_err() as it is called now.
> Thats why I suggested to do a proper shutdown and restart of the tx path to
> avoid the race.

I actually did experiment with adding locking there, too, and no, no
luck. It seems stmmac_tx_err() is more broken than just locking.

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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