netdev - [PATCH 2.6.20-rc3]: 8139cp: Don't blindly enable interrupts

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Message-ID: <45AD4698.7050908@redhat.com>
Date:	Tue, 16 Jan 2007 16:41:44 -0500
From:	Chris Lalancette <clalance@...hat.com>
To:	jgarzik@...ox.com
CC:	Francois Romieu <romieu@...zoreil.com>, netdev@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: [PATCH 2.6.20-rc3]: 8139cp: Don't blindly enable interrupts

Francois Romieu wrote:

>Chris Lalancette <clalance@...hat.com> :
>[...]
>  
>
>>     Thanks for the comments.  While the patch you sent will help, there are
>>still other places that will have problems.  For example, in netpoll_send_skb,
>>we call local_irq_save(flags), then call dev->hard_start_xmit(), and then call
>>local_irq_restore(flags).  This is a similar situation to what I described
>>above; we will re-enable interrupts in cp_start_xmit(), when netpoll_send_skb
>>doesn't expect that, and will probably run into issues.
>>     Is there a problem with changing cp_start_xmit to use the
>>spin_lock_irqsave(), besides the extra instructions it needs?
>>    
>>
>
>No. Given the history of locking in netpoll and the content of
>Documentation/networking/netdevices.txt, asking Herbert which rule(s)
>the code is supposed to follow seemed safer to me.
>
>You can forget my patch.
>
>Please resend your patch inlined to Jeff as described in
>http://linux.yyz.us/patch-format.html.
>
>  
>
Francois,
     Great.  Resending mail, shortening subject to < 65 characters and
inlining the patch.

Thanks,
Chris Lalancette

Similar to this commit:

http://kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=d15e9c4d9a75702b30e00cdf95c71c88e3f3f51e

It's not safe in cp_start_xmit to blindly call spin_lock_irq and then
spin_unlock_irq, since it may very well be the case that cp_start_xmit
was called with interrupts already disabled (I came across this bug in
the context of netdump in RedHat kernels, but the same issue holds, for
example, in netconsole). Therefore, replace all instances of
spin_lock_irq and spin_unlock_irq with spin_lock_irqsave and
spin_unlock_irqrestore, respectively, in cp_start_xmit(). I tested this
against a fully-virtualized Xen guest using netdump, which happens to
use the 8139cp driver to talk to the emulated hardware. I don't have a
real piece of 8139cp hardware to test on, so someone else will have to
do that.

Signed-off-by: Chris Lalancette <clalance@...hat.com>

diff --git a/drivers/net/8139cp.c b/drivers/net/8139cp.c
index e2cb19b..6f93a76 100644
--- a/drivers/net/8139cp.c
+++ b/drivers/net/8139cp.c
@@ -765,17 +765,18 @@ static int cp_start_xmit (struct sk_buff *skb, struct net_device *dev)
 	struct cp_private *cp = netdev_priv(dev);
 	unsigned entry;
 	u32 eor, flags;
+	unsigned long intr_flags;
 #if CP_VLAN_TAG_USED
 	u32 vlan_tag = 0;
 #endif
 	int mss = 0;
 
-	spin_lock_irq(&cp->lock);
+	spin_lock_irqsave(&cp->lock, intr_flags);
 
 	/* This is a hard error, log it. */
 	if (TX_BUFFS_AVAIL(cp) <= (skb_shinfo(skb)->nr_frags + 1)) {
 		netif_stop_queue(dev);
-		spin_unlock_irq(&cp->lock);
+		spin_unlock_irqrestore(&cp->lock, intr_flags);
 		printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
 		       dev->name);
 		return 1;
@@ -908,7 +909,7 @@ static int cp_start_xmit (struct sk_buff *skb, struct net_device *dev)
 	if (TX_BUFFS_AVAIL(cp) <= (MAX_SKB_FRAGS + 1))
 		netif_stop_queue(dev);
 
-	spin_unlock_irq(&cp->lock);
+	spin_unlock_irqrestore(&cp->lock, intr_flags);
 
 	cpw8(TxPoll, NormalTxPoll);
 	dev->trans_start = jiffies;


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