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

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Date:   Thu, 8 Dec 2016 23:45:05 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Pavel Machek <pavel@....cz>
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

On 08.12.2016 23:18, Pavel Machek wrote:
> On Thu 2016-12-08 23:12:10, Lino Sanfilippo wrote:
>> Hi,
>> 
>> On 08.12.2016 22:54, Pavel Machek wrote:
>> > On Thu 2016-12-08 21:32:12, Lino Sanfilippo wrote:
>> >> Hi,
>> >> 
>> >> On 08.12.2016 00:15, Francois Romieu wrote:
>> >> > Lino Sanfilippo <LinoSanfilippo@....de> :
>> >> >> The driver uses a private lock for synchronization between 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 xmit completion handler first takes the private lock
>> >> >> and (in case that the tx queue has been stopped) the xmit_lock, leading
>> >> >> to a reverse locking order and the potential danger of a deadlock.
>> >> > 
>> >> > netif_tx_stop_queue is used by:
>> >> > 1. xmit function before releasing lock and returning.
>> >> > 2. sxgbe_restart_tx_queue()
>> >> >    <- sxgbe_tx_interrupt
>> >> >    <- sxgbe_reset_all_tx_queues()
>> >> >       <- sxgbe_tx_timeout()
>> >> > 
>> >> > Given xmit won't be called again until tx queue is enabled, it's not clear
>> >> > how a deadlock could happen due to #1.
>> >> > 
>> >> 
>> >> 
>> >> After spending more thoughts on this I tend to agree with you. Yes, we have the
>> >> different locking order for the xmit_lock and the private lock in two concurrent
>> >> threads. And one of the first things one learns about locking is that this is a
>> >> good way to create a deadlock sooner or later. But in our case the deadlock 
>> >> can only occur if the xmit function and the tx completion handler perceive different
>> >>  states for the tx queue, or to be more specific: 
>> >> the completion handler sees the tx queue in state "stopped" while the xmit handler 
>> >> sees it in state "running" at the same time. Only then both functions would try to
>> >> take both locks, which could lead to a deadlock.
>> >> 
>> >> OTOH Pavel said that he actually could produce a deadlock. Now I wonder if this is caused
>> >> by that locking scheme (in a way I have not figured out yet) or if it is a different issue.
>> > 
>> > Pavel has some problems, but that's on different hardware.. and it is
>> > possible that it is deadlock (or something else) somewhere else.
>> > 
>> 
>> Right, it is different hardware. But the locking situation in xmit function and tx completion handler
>> is very similar in both drivers. So if a deadlock is not possible in sxgbe it should 
>> also not be possible in stmmac (at least not due to the different locking order). 
>> So maybe there is no real issue that we could fix with removing the private lock and we should
>> keep it as it is.
> 
> Well.. the locking is pretty confused there. Having private lock that
> mirrors lock from network layer is confusing and ugly... that should
> be reason to fix it.
> 									Pavel
> 

Ok. Then I will resend the patches for both drivers with a different (less dramatic) commit message
in which the change is not longer described as a fix for deadlock but rather as a code 
cleanup/improvement, ok?

Regards,
Lino