Subject: Re: [Kgdb-bugreport] [PATCH 2.6.20-rc7] 8139too KGDBoE fix

From: Sergei Shtylyov <[email-masked]>
Date: Wed, 14 Mar 2007 17:04:40 +0300

Hello, I wrote:

>> This thread came up on kgdb-bugreport mailing list. Could you please 
>> suggest us what's the correct way of fixing this problem?

>> 1. When running a kgdb on RTL8139 ethernet interface: 8139too driver 
>> prints too many "Out-of-sync dirty pointer" messages on console and 
>> gdb can't connect to kgdb stub. These messages can be suppressed, 
>> though it still results in connection failures frequently. 

>> 2. Here is how kgdb uses polling mechanism for communication to gdb.  
>> kgdb calls netpoll_set_trap(1) just before entering a loop where it 
>> communicates to gdb. It calls netpoll_set_trap(0) after it is done and 
>> wants to resume a kernel. The communication to gdb goes through 
>> netpoll_poll (which calls kgdb rx_hook) and netpoll_send_udp functions.

>> 3. A queue for an interface may have been stopped by it's driver by 
>> calling netif_stop_queue. After this if kgdb attempts to enter 
>> communication with gdb, it'll call netpoll_set_trap(1), after which 
>> the queue can't be started again. This is a potential deadlock 
>> situation. Is there a way out of this?

>> 4. Is it necessary to call netpoll_set_trap(1) at all before entering 
>> gdb communication loop? Even if a driver stops the queue in middle of 
>> the communication netpoll_poll and netpoll_send_udp calls can recover 
>> from that by calling driver's interrupt and poll routines. Is this a 
>> valid statement?

>    I'd like to return to this again (having received no feedback)...
>    The idea is to change how CONFIG_NETPOLL_TRAP is implemented: instead of
> completely bypassing queue locking after netpoll_set_trap(1) has been 
> called, how about we set and chack some other flag (internal to netpoll) 
> telling it that the queue is frozen, i.e. watch the queue state using a 
> separate mechanism when traffic trapping is engaged?  This certainly 

    Well, this certainly won't work, as the bit should be tied to struct 
net_device.  The first idea was more sound: just set/reset __LINK_STATE_XOFF 
flag, not calling __netif_schedule(), i.e. remove #ifdef from 
netif_stop_queue() and replace return stmt in netif_wake_queue() by 
clear_bit(__LINK_STATE_XOFF, &dev->state).

> would avoid TX queue overflows in drivers while also avoiding any 
> dev->state changes and even worse evil __netif_schedule() call, i.e. 
> things that CONFIG_NETPOLL_TRAP is currectly trying to avoid, AFAIU...

    I think I'll submit a patch -- netpoll traffic trapping is pretty broken 
as it is now.

>> Thanks a lot.
>> -Amit

WBR, Sergei