netdev - net_tx_action race condition?

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Message-ID: <547332D7.7050104@3dautomazione.it>
Date:	Mon, 24 Nov 2014 14:29:59 +0100
From:	Angelo Rizzi <angelo.rizzi@...utomazione.it>
To:	netdev@...r.kernel.org
Subject: net_tx_action race condition?

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

View attachment "net_tx_action_patch" of type "text/plain" (439 bytes)