Re: PROBLEM: kernel oops with g_serial USB gadget on 2.6.30

[Alan Cox]

> [   55.630000] [<c0274e74>] (_spin_lock_irqsave+0x44/0x58) from [<c019d608>] (gs_write_room+0x10/0x58)
> [   55.630000] [<c019d608>] (gs_write_room+0x10/0x58) from [<c0156058>] (tty_write_room+0x20/0x28)
> [   55.630000] [<c0156058>] (tty_write_room+0x20/0x28) from [<c01538e0>] (process_echoes+0x4c/0x288)
> [   55.630000] [<c01538e0>] (process_echoes+0x4c/0x288) from [<c0155a40>] (n_tty_receive_buf+0x9ec/0xecc)
> [   55.630000] [<c0155a40>] (n_tty_receive_buf+0x9ec/0xecc) from [<c0158174>] (flush_to_ldisc+0x104/0x1b0)
> [   55.630000] [<c0158174>] (flush_to_ldisc+0x104/0x1b0) from [<c00498e0>] (worker_thread+0x1d0/0x2cc)
> [   55.630000] [<c00498e0>] (worker_thread+0x1d0/0x2cc) from [<c004d55c>] (kthread+0x58/0x90)
> [   55.630000] [<c004d55c>] (kthread+0x58/0x90) from [<c003c03c>] (do_exit+0x0/0x5d0)
> [   55.630000] [<c003c03c>] (do_exit+0x0/0x5d0) from [<c6c26180>] (0xc6c26180)
> [   55.630000] Code: ea000076 e59d100c e3510000 1a000002 (e5994004)
> [   55.640000] ---[ end trace f9a4499d9482c504 ]---
> [   55.650000] note: events/0[5] exited with preempt_count 1
> 
> The low latency mode is disabled. I've tested different ways of gathering
> the debug output from my lowlevel udc driver: in first attempt it was 
> printk(KERN_INFO ...) then I switched to use lowlevel printascii() to remove
> possible interaction with console/tty framework at all. In both cases I got
> the above NULL pointer exception. It only worked fine when I switched to
> printk(KERN_DEBUG ...) with debug level above KERN_DEBUG, so all the
> messages were only buffered in dmesg buffer. This looks like a timing issue
> in tty framework...

Looks to me like the gs_write_room function takes a lock already held
when the driver calls tty_flip_buffer_push() - which will and can call
back into the driver (eg for echo processing)
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