On Mon, Jan 28, 2019 at 09:21:31AM -0800, Doug Anderson wrote:
Hi, On Mon, Jan 28, 2019 at 9:16 AM Sasha Levin <sashal@xxxxxxxxxx> wrote:On Mon, Jan 28, 2019 at 08:05:13AM -0800, Doug Anderson wrote: >Hi, > >On Mon, Jan 28, 2019 at 7:44 AM Sasha Levin <sashal@xxxxxxxxxx> wrote: >> >> From: Douglas Anderson <dianders@xxxxxxxxxxxx> >> >> [ Upstream commit d6e1935819db0c91ce4a5af82466f3ab50d17346 ] >> >> Right now serial drivers process sysrq keys deep in their character >> receiving code. This means that they've already grabbed their >> port->lock spinlock. This can end up getting in the way if we've go >> to do serial stuff (especially kgdb) in response to the sysrq. >> >> Serial drivers have various hacks in them to handle this. Looking at >> '8250_port.c' you can see that the console_write() skips locking if >> we're in the sysrq handler. Looking at 'msm_serial.c' you can see >> that the port lock is dropped around uart_handle_sysrq_char(). >> >> It turns out that these hacks aren't exactly perfect. If you have >> lockdep turned on and use something like the 8250_port hack you'll get >> a splat that looks like: >> >> WARNING: possible circular locking dependency detected >> [...] is trying to acquire lock: >> ... (console_owner){-.-.}, at: console_unlock+0x2e0/0x5e4 >> >> but task is already holding lock: >> ... (&port_lock_key){-.-.}, at: serial8250_handle_irq+0x30/0xe4 >> >> which lock already depends on the new lock. >> >> the existing dependency chain (in reverse order) is: >> >> -> #1 (&port_lock_key){-.-.}: >> _raw_spin_lock_irqsave+0x58/0x70 >> serial8250_console_write+0xa8/0x250 >> univ8250_console_write+0x40/0x4c >> console_unlock+0x528/0x5e4 >> register_console+0x2c4/0x3b0 >> uart_add_one_port+0x350/0x478 >> serial8250_register_8250_port+0x350/0x3a8 >> dw8250_probe+0x67c/0x754 >> platform_drv_probe+0x58/0xa4 >> really_probe+0x150/0x294 >> driver_probe_device+0xac/0xe8 >> __driver_attach+0x98/0xd0 >> bus_for_each_dev+0x84/0xc8 >> driver_attach+0x2c/0x34 >> bus_add_driver+0xf0/0x1ec >> driver_register+0xb4/0x100 >> __platform_driver_register+0x60/0x6c >> dw8250_platform_driver_init+0x20/0x28 >> ... >> >> -> #0 (console_owner){-.-.}: >> lock_acquire+0x1e8/0x214 >> console_unlock+0x35c/0x5e4 >> vprintk_emit+0x230/0x274 >> vprintk_default+0x7c/0x84 >> vprintk_func+0x190/0x1bc >> printk+0x80/0xa0 >> __handle_sysrq+0x104/0x21c >> handle_sysrq+0x30/0x3c >> serial8250_read_char+0x15c/0x18c >> serial8250_rx_chars+0x34/0x74 >> serial8250_handle_irq+0x9c/0xe4 >> dw8250_handle_irq+0x98/0xcc >> serial8250_interrupt+0x50/0xe8 >> ... >> >> other info that might help us debug this: >> >> Possible unsafe locking scenario: >> >> CPU0 CPU1 >> ---- ---- >> lock(&port_lock_key); >> lock(console_owner); >> lock(&port_lock_key); >> lock(console_owner); >> >> *** DEADLOCK *** >> >> The hack used in 'msm_serial.c' doesn't cause the above splats but it >> seems a bit ugly to unlock / lock our spinlock deep in our irq >> handler. >> >> It seems like we could defer processing the sysrq until the end of the >> interrupt handler right after we've unlocked the port. With this >> scheme if a whole batch of sysrq characters comes in one irq then we >> won't handle them all, but that seems like it should be a fine >> compromise. >> >> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> >> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> >> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> >> --- >> include/linux/serial_core.h | 37 ++++++++++++++++++++++++++++++++++++- >> 1 file changed, 36 insertions(+), 1 deletion(-) > >FWIW this patch shouldn't hurt to be backported (I haven't heard any >problems report with it), but it is effectively a no-op unless you >also pick a patch that uses the new API. For instance commit >596f63da42b9 ("serial: 8250: Process sysrq at port unlock time"). >...and if you want that patch I think you also need commit >3e6f88068314 ("serial: core: Include console.h from serial_core.h"). > >In theory you could think about adding the "qcom_geni_serial" patches >related to sysrq processing too--dunno if anyone really cares about >those on 4.20 stable... Since no one actually tagged it for stable, probably not... I'll drop it, thanks!OK. Whatever behavior you decide on, please apply it across the board. I got pings that this same patch was being picked to lots and lots of different stable kernels and it is equally a no-op (without the followup patches) everywhere.
Yup, I send a mail for each branch it was added on. It was now dropped from all of them. -- Thanks, Sasha
