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

Sasha Levin
2019-01-28T17:16:20Z

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!

--
Thanks,
Sasha