Re: [PATCH] niu: fix SMP race protecting rx_rings and tx_rings

[David Miller <davem@davemloft.net>]

From: Flavio Leitner <fleitner@redhat.com>
Date: Thu,  3 Feb 2011 16:45:17 -0200

> It's possible to trigger a crash if one CPU is opening
> the device while another CPU gets its statistics.
> 
> It happens because niu_alloc_channels() called from
> niu_open() sets num_tx/rx_rings before allocating the
> ring, so the other thread crashes when accessing
> np->tx_rings[i].tx_packets at niu_get_tx_stats().
> 
> Signed-off-by: Flavio Leitner <fleitner@redhat.com>
 ...
> Compile tested only because I don't have the hardware.

I'll apply this and give it a quick test, thanks.

Can you have the person who reported this crash to you test the patch
out at least?  That's how you learned about this problem, right,
someone else hit the crash?

In such cases I'd really appreciate it if you got positive testing
feedback from the reporter before posting the patch.

Longer term a better way to fix this is to RCU free the ring data
structures, and use a quick NULL test at the top of the get stats
implementation.