linux-kernel - Re: [PATCH net v2 6/8] forcedeth: Fix a race during rmmod of forcedeth

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Message-ID: <CAG88wWZdRZnm8QtuhWtkc0dQprDK=7+8NifvUYYtOfQE9KKBVA@mail.gmail.com>
Date:	Fri, 4 Nov 2011 10:22:40 -0700
From:	David Decotigny <david.decotigny@...gle.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Ian Campbell <ian.campbell@...rix.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Jiri Pirko <jpirko@...hat.com>, Joe Perches <joe@...ches.com>,
	Szymon Janc <szymon@...c.net.pl>,
	Salman Qazi <sqazi@...gle.com>
Subject: Re: [PATCH net v2 6/8] forcedeth: Fix a race during rmmod of forcedeth

Ben,

Thank you for your comments. I understand this patch needs more work.
So I am going to remove it from this series for now and work on it in
isolation.

Regards,

On Thu, Nov 3, 2011 at 8:46 PM, Ben Hutchings <bhutchings@...arflare.com> wrote:
> On Thu, 2011-11-03 at 18:41 -0700, David Decotigny wrote:
>> From: Salman Qazi <sqazi@...gle.com>
>>
>> The race was between del_timer_sync and nv_do_stats_poll called through
>> nv_get_ethtool_stats.
>
> I don't think so.  nv_close() and nv_get_ethtool_stats() are both called
> with RTNL held.
>
> Calling the timer function from nv_get_ethtool_stats is very likely part
> of the problem though, so why don't you stop doing that?
>
> [...]
>> diff --git a/drivers/net/ethernet/nvidia/forcedeth.c b/drivers/net/ethernet/nvidia/forcedeth.c
>> index 0af12a8..7996782 100644
>> --- a/drivers/net/ethernet/nvidia/forcedeth.c
>> +++ b/drivers/net/ethernet/nvidia/forcedeth.c
>> @@ -3937,6 +3937,10 @@ static void nv_poll_controller(struct net_device *dev)
>>  }
>>  #endif
>>
>> +/* No locking is needed as long as this is in the timer
>> + * callback.  However, any other callers must call this
>> + * function with np->lock held.
>> + */
>
> So long as this function is used by all of (1) the timer function (2)
> the ndo_get_stats implementation (3) the ethtool get_stats
> implementation, it can most certainly be called concurrently on multiple
> processors.
>
> You could have (2) and (3) return the last polled stats and not poll the
> hardware themselves, but you would need to use the functions from
> <linux/u64_stats_sync.h> to avoid word-tearing on 32-bit architectures.
>
>>  static void nv_do_stats_poll(unsigned long data)
>>  {
>>         struct net_device *dev = (struct net_device *) data;
>> @@ -4589,12 +4593,17 @@ static int nv_get_sset_count(struct net_device *dev, int sset)
>>
>>  static void nv_get_ethtool_stats(struct net_device *dev, struct ethtool_stats *estats, u64 *buffer)
>>  {
>> +       unsigned long flags;
>>         struct fe_priv *np = netdev_priv(dev);
>>
>> +       spin_lock_irqsave(&np->lock, flags);
>> +
>>         /* update stats */
>>         nv_do_stats_poll((unsigned long)dev);
>>
>>         memcpy(buffer, &np->estats, nv_get_sset_count(dev, ETH_SS_STATS)*sizeof(u64));
>> +
>> +       spin_unlock_irqrestore(&np->lock, flags);
>
> This function is not called from interrupt context.
>
>>  }
>>
>>  static int nv_link_test(struct net_device *dev)
>> @@ -5189,13 +5198,13 @@ static int nv_close(struct net_device *dev)
>>
>>         spin_lock_irq(&np->lock);
>>         np->in_shutdown = 1;
>> +       del_timer_sync(&np->stats_poll);
>>         spin_unlock_irq(&np->lock);
>>         nv_napi_disable(dev);
>>         synchronize_irq(np->pci_dev->irq);
>>
>>         del_timer_sync(&np->oom_kick);
>>         del_timer_sync(&np->nic_poll);
>> -       del_timer_sync(&np->stats_poll);
>>
>>         netif_stop_queue(dev);
>>         spin_lock_irq(&np->lock);
>
> I don't believe this code movement is helpful.
>
> Ben.
>
> --
> Ben Hutchings, Staff Engineer, Solarflare
> Not speaking for my employer; that's the marketing department's job.
> They asked us to note that Solarflare product names are trademarked.
>
>
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