netdev - Re: net_tx_action race condition?

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Date:	Mon, 24 Nov 2014 07:33:20 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Angelo Rizzi <angelo.rizzi@...utomazione.it>
Cc:	netdev@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: net_tx_action race condition?

On Mon, 2014-11-24 at 14:29 +0100, Angelo Rizzi wrote:
> Hi Daniel,
> Here attached the patch file you required.
> The problem i've found is on the declaration of 'struct softnet_data 
> *sd' in function 'net_tx_action'
> What happens to me (i have an embedded system based on FPGA and a NIOS2 
> cpu) is that, due to compiler optimization, the content of 
> 'sd->completion_queue' is saved in a CPU register before interrupt 
> disabling (when the instruction 'if (sd->completion_queue) {' is 
> executed) and then the register contents is used for interrupt-disabled 
> assignment ('clist = sd->completion_queue') instead of re-read the 
> variable contents.
> This seems to lead to a race condition when an interrupt modifies the 
> content of 'sd->completion_queue' between these two instructions.
> What i have done to avoid this situation is to change the declaration of 
> 'struct softnet_data *sd' to 'volatile struct softnet_data *sd' and now 
> everything seems to be ok.
> I hope this will help.
> 
> Regards,
> Angelo
> 

Do not add volatile in the kernel, this is not how we solve this kind of
problems.  Documentation/memory-barriers.txt and

Documentation/00-INDEX:476:volatile-considered-harmful.txt
Documentation/00-INDEX:477:     - Why the "volatile" type class should not be used


I am surprised this patch is needed. Many other 'bugs' would need
similar fixes.

local_irq_disable() MUST have a memory barrier. This looks like a bug in
one particular arch implementation.


On x86 for example, native_irq_disable() is really :

	asm volatile("cli": : :"memory");


diff --git a/net/core/dev.c b/net/core/dev.c
index ac4836241a965a71952469ba054f87d8dfca2d32..fa73072e515aa07fa8ae1bc39174b7d59c7a31a5 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3406,7 +3406,7 @@ static void net_tx_action(struct softirq_action *h)
 		struct sk_buff *clist;
 
 		local_irq_disable();
-		clist = sd->completion_queue;
+		clist = ACCESS_ONCE(sd->completion_queue);
 		sd->completion_queue = NULL;
 		local_irq_enable();
 


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