netdev - Re: net_tx_action race condition?

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Message-ID: <20141124150832.GA32184@gondor.apana.org.au>
Date:	Mon, 24 Nov 2014 23:08:32 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Angelo Rizzi <angelo.rizzi@...utomazione.it>
Cc:	netdev@...r.kernel.org
Subject: Re: net_tx_action race condition?

Angelo Rizzi <angelo.rizzi@...utomazione.it> wrote:
> [-- text/plain, encoding 7bit, charset: iso-8859-15, 22 lines --]
> 
> 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.

local_irq_disable is supposed to contain a compiler barrier.  So
this is either a buggy arch or a buggy compiler.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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