linux-kernel - Re: [PATCH v2 2/2] net: ethernet: stmmac: remove private tx queue lock

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Date:   Thu, 15 Dec 2016 20:37:03 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Pavel Machek <pavel@....cz>
Cc:     bh74.an@...sung.com, ks.giri@...sung.com, vipul.pandya@...sung.com,
        peppe.cavallaro@...com, alexandre.torgue@...com,
        romieu@...zoreil.com, davem@...emloft.net,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2 2/2] net: ethernet: stmmac: remove private tx queue
 lock

Hi,

On 15.12.2016 10:45, Pavel Machek wrote:
> Hi!
> 
>> The driver uses a private lock for synchronization of the xmit function and
>> the xmit completion handler, but since the NETIF_F_LLTX flag is not set,
>> the xmit function is also called with the xmit_lock held.
>> 
>> On the other hand the completion handler uses the reverse locking order by
>> first taking the private lock and (in case that the tx queue had been
>> stopped) then the xmit_lock.
>> 
>> Improve the locking by removing the private lock and using only the
>> xmit_lock for synchronization instead.
> 
> Do you have stmmac hardware to test on?
> 

Unfortunately not (I mentioned that the patch I send was only compile tested in 
the first version but I think I forgot to do so in the last version).

> I believe something is very wrong with the locking there. In
> particular... scheduling the stmmac_tx_timer() function to run often
> should not do anything bad if locking is correct... but it breaks the
> driver rather quickly. [Example patch below, needs applying to two
> places in net-next.]
> 

Do you get this result only after the private lock is removed? Or has this problem
been there before? And how exactly does the failure look like?

Regards,
Lino