Re: [PATCH AUTOSEL 4.20 035/304] serial: core: Allow processing sysrq at port unlock time

Doug Anderson
2019-01-28T17:21:58Z

Hi,

On Mon, Jan 28, 2019 at 9:16 AM Sasha Levin <sashal@kernel.org> 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@kernel.org> wrote:
> >>
> >> From: Douglas Anderson <dianders@chromium.org>
> >>
> >> [ 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@chromium.org>
> >> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> >> Signed-off-by: Sasha Levin <sashal@kernel.org>
> >> ---
> >> 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.

-Doug