Re: [PATCH v2 2/2] Driver for the Atmel USBA on-chip UDC

I tripped over a locking bug when I tried this:

BUG: spinlock recursion on CPU#0, swapper/0
 lock: 901d2b18, .magic: dead4ead, .owner: swapper/0, .owner_cpu: 0
Call trace:
 [<900134b0>] dump_stack+0x18/0x20
 [<900bb792>] spin_bug+0x6a/0x7c
 [<900bb93a>] _raw_spin_lock+0x26/0xd0
 [<90141a8a>] _spin_lock_irqsave+0xe/0x14
 [<900dee2a>] usba_ep_queue+0x10e/0x198
 [<c08780c0>] zero_setup+0x3b4/0x40c [g_zero]
 [<900df618>] usba_udc_irq+0x72e/0x8c6
 [<9002ffcc>] handle_IRQ_event+0x1e/0x42
 [<90030fc2>] handle_simple_irq+0x66/0x94
 [<90011356>] do_IRQ+0x2e/0x54
 [<900104fe>] irq_level0+0x18/0x5e
 [<901401be>] rest_init+0x3a/0x48
 [<90000570>] start_kernel+0x1e4/0x214
 [<00000000>] 0x0

BUG: spinlock lockup on CPU#0, swapper/0, 901d2b18
Call trace:
 [<900134b0>] dump_stack+0x18/0x20
 [<900bb9b8>] _raw_spin_lock+0xa4/0xd0
 [<90141a8a>] _spin_lock_irqsave+0xe/0x14
 [<900dee2a>] usba_ep_queue+0x10e/0x198
 [<c08780c0>] zero_setup+0x3b4/0x40c [g_zero]
 [<900df618>] usba_udc_irq+0x72e/0x8c6
 [<9002ffcc>] handle_IRQ_event+0x1e/0x42
 [<90030fc2>] handle_simple_irq+0x66/0x94
 [<90011356>] do_IRQ+0x2e/0x54
 [<900104fe>] irq_level0+0x18/0x5e
 [<901401be>] rest_init+0x3a/0x48
 [<90000570>] start_kernel+0x1e4/0x214
 [<00000000>] 0x0

This happened when I plugged in a gadget zero peripheral, but
given the nature of this bug I'd expect it to show up with every
gadget driver.

Every time you call out from the UDC driver to a gadget driver,
you need to drop the lock ... so that it can call back in to the
driver, without causing lock recursion bugs like the one above.

(Also, it's good practice to run with memory poisoning and lock
debugging enabled all the time if you're a developer ... you never
know when you'll accidentally insert a bug, and it's easier to
track them down if you can at least say that "it must have been
one of the changes I made this morning", while those changes are
fresh on your mind!)

- Dave


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