Re: [PATCH] serial: Add spin_lock_init() in 8250 early_serial_setup() to init port.lock

[George G. Davis]

On Tue, Jan 31, 2006 at 12:39:27AM +0900, Atsushi Nemoto wrote:
> >>>>> On Wed, 25 Jan 2006 22:24:03 -0500, "George G. Davis" <gdavis@xxxxxxxxxx> said:
> 
> gdavis> Need spin_lock_init(&serial8250_ports[port->line].port.lock)
> gdavis> in early_serial_setup() since we're copying struct uart_port
> gdavis> *port into serial8250_ports[port->line].port and *port.lock is
> gdavis> typically unitiliased by the caller.
> 
> Is this really needed?

Yes, IMO, it is required for targets which use early_serial_setup().  If
you have doubts, enable DEBUG_SPINLOCK and you will see the problem but
only if your target uses early_serial_setup().

> The port.lock will be initialized in
> uart_set_options()

uart_set_options() is only called from serial8250_console_setup() when 8250
serial devices have already been registered via serial8250_isa_init_ports()
(for legacy devices only though AFAICT).

> or uart_add_one_port().

But uart_add_one_port() intentionally does not spin_lock_init() the
port.lock of the serial console device under the assumption that
it is already done.


Here's the call sequence:


start_kernel()
	...
	console_init()
		...
		serial8250_console_init()
			serial8250_isa_init_ports()
				if (first)
					spin_lock_init() /* port.lock init */
				...

All 8250 serial port.locks are now initialised but no ports have
been registered (on targets which do not register legacy serial ports
via old_serial_port[]) at this point.


			register_console()
				serial8250_console_setup() /* -ENODEV */
	...
	rest_init()
		...
		/* arch_initcalls */
		early_serial_setup()
			serial8250_isa_init_ports() /* !first, do nothing */
			serial8250_ports[port->line].port = *port;
			...

serial8250_ports[port->line].port.lock is over written above and may
not be properly initialised.


		/* module_inits */
		serial8250_init()
			...
			serial8250_register_ports()
				serial8250_isa_init_ports() /* !first */
				uart_add_one_port()
					if (!uart_console(port))
						spin_lock_init(&port->lock);
					uart_configure_port()


At this point I see the following with DEBUG_SPINLOCK enabled:


Serial: 8250/16550 driver $Revision: 1.90 $ 4 ports, IRQ sharing disabled
BUG: spinlock bad magic on CPU#0, swapper/1
 lock: c02fcffc, .magic: 00000000, .owner: <none>/-1, .owner_cpu: 0
[<c002bdec>] (dump_stack+0x0/0x14) from [<c0136268>] (spin_bug+0x94/0xac)
[<c01361d4>] (spin_bug+0x0/0xac) from [<c01362a8>] (_raw_spin_lock+0x28/0x160)
 r5 = A0000013  r4 = C02FCFFC 
[<c0136280>] (_raw_spin_lock+0x0/0x160) from [<c024459c>] (_spin_lock_irqsave+0x2c/0x34)
 r8 = 00000000  r7 = C02FCFFC  r6 = C02A4144  r5 = A0000013
 r4 = C02FCFFC 
[<c0244570>] (_spin_lock_irqsave+0x0/0x34) from [<c0168748>] (serial8250_config_port+0x7c/0x988)
 r5 = C02FCFFC  r4 = C02FCFFC 
[<c01686cc>] (serial8250_config_port+0x0/0x988) from [<c01668cc>] (uart_add_one_port+0x110/0x2a0)
[<c01667bc>] (uart_add_one_port+0x0/0x2a0) from [<c001a174>] (serial8250_init+0xec/0x170)
 r8 = C02A40F4  r7 = C1C7B2B0  r6 = C02FD2FC  r5 = 00000000
 r4 = C02FCFFC 
[<c001a088>] (serial8250_init+0x0/0x170) from [<c0026110>] (init+0xa0/0x228)
 r8 = 00000000  r7 = C00205B4  r6 = 00000001  r5 = C0374000
 r4 = C0020504 
[<c0026070>] (init+0x0/0x228) from [<c00446cc>] (do_exit+0x0/0x874)
 r7 = 00000000  r6 = 00000000  r5 = 00000000  r4 = 00000000
BUG: spinlock lockup on CPU#0, swapper/1, c02fcffc
[<c002bdec>] (dump_stack+0x0/0x14) from [<c01363a8>] (_raw_spin_lock+0x128/0x160)
[<c0136280>] (_raw_spin_lock+0x0/0x160) from [<c024459c>] (_spin_lock_irqsave+0x2c/0x34)
 r8 = 00000000  r7 = C02FCFFC  r6 = C02A4144  r5 = A0000013
 r4 = C02FCFFC 
[<c0244570>] (_spin_lock_irqsave+0x0/0x34) from [<c0168748>] (serial8250_config_port+0x7c/0x988)
 r5 = C02FCFFC  r4 = C02FCFFC 
[<c01686cc>] (serial8250_config_port+0x0/0x988) from [<c01668cc>] (uart_add_one_port+0x110/0x2a0)
[<c01667bc>] (uart_add_one_port+0x0/0x2a0) from [<c001a174>] (serial8250_init+0xec/0x170)
 r8 = C02A40F4  r7 = C1C7B2B0  r6 = C02FD2FC  r5 = 00000000
 r4 = C02FCFFC 
[<c001a088>] (serial8250_init+0x0/0x170) from [<c0026110>] (init+0xa0/0x228)
 r8 = 00000000  r7 = C00205B4  r6 = 00000001  r5 = C0374000
 r4 = C0020504 
[<c0026070>] (init+0x0/0x228) from [<c00446cc>] (do_exit+0x0/0x874)
 r7 = 00000000  r6 = 00000000  r5 = 00000000  r4 = 00000000


Adding spin_lock_init() in early_serial_setup() resolves the above.  Perhaps
there is a better way to fix this?

> 
> I think spin_lock_init() in serial8250_isa_init_ports() can be omitted
> also.

AFAICS, there are still a few corner cases where port.lock's are not
initialised on targets which do not define legacy serial ports via
old_serial_port[].


Thanks!

--
Regards,
George
> 
> ---
> Atsushi Nemoto
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