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

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Date:   Mon, 28 Jan 2019 08:05:13 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH AUTOSEL 4.20 035/304] serial: core: Allow processing sysrq
 at port unlock time

Hi,

On Mon, Jan 28, 2019 at 7:44 AM Sasha Levin <sashal@...nel.org> wrote:
>
> From: Douglas Anderson <dianders@...omium.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@...omium.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Sasha Levin <sashal@...nel.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