Re: Regression in fd5f7cde1b85 ("printk: Never set console_may_schedule in console_trylock()")

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (09/17/19 16:10), Uwe Kleine-König wrote:
> Hello,
>
> Today it saw sysrq on an UART driven by drivers/tty/serial/imx.c report
> a lockdep issue. Bisecting pointed to
>
> 	fd5f7cde1b85 ("printk: Never set console_may_schedule in console_trylock()")

Hmmm...

I don't see how this patch can affect anything. It simply
disables preemption in printk().

> When I type <break>t I get:
> 
> [   87.940104] sysrq: SysRq : This sysrq operation is disabled.
> [   87.948752] 
> [   87.948772] ======================================================
> [   87.948787] WARNING: possible circular locking dependency detected
> [   87.948798] 4.14.0-12954-gfd5f7cde1b85 #26 Not tainted
> [   87.948813] ------------------------------------------------------
> [   87.948822] swapper/0 is trying to acquire lock:
> [   87.948829]  (console_owner){-...}, at: [<c015e438>] console_unlock+0x110/0x598
> [   87.948861] 
> [   87.948869] but task is already holding lock:
> [   87.948874]  (&port_lock_key){-.-.}, at: [<c048d5b0>] imx_rxint+0x2c/0x290
> [   87.948902] 
> [   87.948911] which lock already depends on the new lock.
> [   87.948917] 
> [   87.948923] 
> [   87.948932] the existing dependency chain (in reverse order) is:
> [   87.948938] 
> [   87.948943] -> #1 (&port_lock_key){-.-.}:
> [   87.948975]        _raw_spin_lock_irqsave+0x5c/0x70
> [   87.948983]        imx_console_write+0x138/0x15c
> [   87.948991]        console_unlock+0x204/0x598
> [   87.949000]        register_console+0x21c/0x3e8
> [   87.949008]        uart_add_one_port+0x3e4/0x4dc
> [   87.949019]        platform_drv_probe+0x3c/0x78
> [   87.949027]        driver_probe_device+0x25c/0x47c
> [   87.949035]        __driver_attach+0xec/0x114
> [   87.949044]        bus_for_each_dev+0x80/0xb0
> [   87.949054]        bus_add_driver+0x1d4/0x264
> [   87.949062]        driver_register+0x80/0xfc
> [   87.949069]        imx_serial_init+0x28/0x48
> [   87.949078]        do_one_initcall+0x44/0x18c
> [   87.949087]        kernel_init_freeable+0x11c/0x1cc
> [   87.949095]        kernel_init+0x10/0x114
> [   87.949103]        ret_from_fork+0x14/0x30

This is "normal" locking path

	console_sem -> port->lock

	printk()
	 lock console_sem
	  imx_console_write()
	   lock port->lock

> [   87.949113] -> #0 (console_owner){-...}:
> [   87.949145]        lock_acquire+0x100/0x23c
> [   87.949154]        console_unlock+0x1a4/0x598
> [   87.949162]        vprintk_emit+0x1a4/0x45c
> [   87.949171]        vprintk_default+0x28/0x30
> [   87.949180]        printk+0x28/0x38
> [   87.949189]        __handle_sysrq+0x1c4/0x244
> [   87.949196]        imx_rxint+0x258/0x290
> [   87.949206]        imx_int+0x170/0x178
> [   87.949216]        __handle_irq_event_percpu+0x78/0x418
> [   87.949225]        handle_irq_event_percpu+0x24/0x6c
> [   87.949233]        handle_irq_event+0x40/0x64
> [   87.949242]        handle_level_irq+0xb4/0x138
> [   87.949252]        generic_handle_irq+0x28/0x3c
> [   87.949261]        __handle_domain_irq+0x50/0xb0
> [   87.949269]        avic_handle_irq+0x3c/0x5c
> [   87.949277]        __irq_svc+0x6c/0xa4
> [   87.949287]        arch_cpu_idle+0x30/0x40
> [   87.949297]        arch_cpu_idle+0x30/0x40
> [   87.949305]        do_idle+0xa0/0x104
> [   87.949313]        cpu_startup_entry+0x14/0x18
> [   87.949323]        start_kernel+0x30c/0x368

This one is a "reverse" locking path...

	port->lock -> console_sem

There is more to it:

 imxint()
  lock port->lock
   uart_handle_sysrq_char()
    handle_sysrq()
     printk()
      lock conosole_sem
       imx_console_write()
        lock port->lock			[boom]

This path re-enters serial driver. But it doesn't deadlock, because
uart_handle_sysrq_char() sets a special flag port->sysrq, and serial
consoles are expected to make sure that they don't lock port->lock
in this case. Otherwise we will kill the system:

	void serial_console_write(...)
	{
	...
          if (sport->port.sysrq)
                  locked = 0;
          else if (oops_in_progress)
                  locked = spin_trylock_irqsave(&sport->port.lock, flags);
          else
                  spin_lock_irqsave(&sport->port.lock, flags);
 	...
	}

So I'd say that lockdep is correct, but there are several hacks which
prevent actual deadlock.

No idea why bisection has pointed at fd5f7cde1b85, it really doesn't
change the locking patterns.

	-ss