On 10/21/2013 11:07 PM, Alexandre Oliva wrote:
It's been at least a year since I first noticed that, on WiFi-busy environments such as airports, hotels and Free Software conferences, my Yeeloong laptop with a RTL8187B WiFi card will freeze or oops shortly after I enable WiFi. This problem doesn't seem to happen when I'm at home, probably because of the low WiFi traffic. The problem occurs while running 3.11.* and 3.10.* kernels, but not 3.4.* or 3.0.*. I couldn't find any changes to the rtl8187 module that explain this misbehavior, so I suspect it's some new source of parallelism in the mac80211 layer that has exposed the lack of synchronization in uses of rx_queue and b_tx_status.queue. Indeed, I found many uses of these queues that don't take locks to ensure consistency. Unfortunately, adding spin locks around all uses causes harder freezes and/or complains about scheduling in atomic contexts, depending on which race I hit first. Without any changes, the problem I get most often is a crash within rtl8187b_status_cb, when skb_unlink attempts to dereference a NULL pointer. Testing skb->prev and skb->next before entering the branch where the skb is removed seemed to make the error a little bit less frequent, but surely not enough for the machine to remain up for very long while WiFi is enabled. Is this a known problem? Any suggestions on what I could try next to fix the problem?
No, the problem has not previously been reported. From your description of the situation where it happens, the problem requires a lot of same channel, same AP traffic. I will try to duplicate that condition here. Although I have an RTL8187B device, I seldom use it as the case on the USB stick is falling apart. I will need to do some repair on it so that it holds together.
After inspecting the code in rtl8187b_status_cb, I did notice that it does a lot of things that should be done by mac80211. As you have been testing code modifications, I assume that you will be able to test any patches that I generate.
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