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SubjectRegression in dbdda842fe96 ("printk: Add console owner and waiter logic to load balance console writes") [Was: Regression in fd5f7cde1b85 ("...")]
Hello Sergey,

On Wed, Sep 18, 2019 at 10:30:32AM +0900, Sergey Senozhatsky wrote:
> 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().

I rechecked and indeed fd5f7cde1b85's parent has the problem, too, so I
did a mistake during my bisection :-|

Redoing the bisection (a bit quicker this time) points to

dbdda842fe96 ("printk: Add console owner and waiter logic to load balance console writes")

Sorry for the confusion.

> > 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.

Just to make sure, I got you right: With the way lockdep works it is
right to assume there is a problem, but in fact there isn't?
This is IMHO unfortunate because such false positives reduces the
usefulness of lockdep considerably. :-|

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

See above. I bent off wrongly during bisection and dbdda842fe96
("printk: Add console owner and waiter logic to load balance console
writes") is the first commit that issues the lockdep splat. I guess that
doesn't change what you said above though.

Best regards
Uwe

--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | http://www.pengutronix.de/ |

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