Messages in this thread | | | Date | Wed, 18 Sep 2019 09:11:58 +0200 | From | Uwe Kleine-König <> | Subject | Regression in dbdda842fe96 ("printk: Add console owner and waiter logic to load balance console writes") [Was: Regression in fd5f7cde1b85 ("...")] |
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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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