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

[Sasha Levin]

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!

--
Thanks,
Sasha