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

Doug Anderson
2019-01-28T17:17:45Z

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...

-Doug