Re: tty_flip_buffer() from atomic context when low_latency==1

[Grant Edwards <grant.b.edwards@gmail.com>]

On 2011-08-19, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> On Fri, 19 Aug 2011 20:44:26 +0000 (UTC)
> Grant Edwards <grant.b.edwards@gmail.com> wrote:
>
>> On 2011-08-19, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
>> 
>> >> But, many drivers appear to call tty_flip_buffer_push() from an atomic
>> >> context. If that is done with tty->low_latency==1 it turns into a call
>> >> to flush_to_ldisc(), which then calls disc->ops->receive_buf(), which
>> >> must not be called from an atomic context.
>> >
>> > It's covered under "So don't do that then".
>> 
>> I don't do that, and wasn't asking if I should.
>> 
>> My point was that most of the drivers in the linux kernels sources
>> _do_ appear to do that, and I don't see how they avoid problems.
>> 
>> For exaple, 8250.c calls tty_flip_buffer_push() from receive_chars(),
>> calld from serial8250_handle_port(), called from
>> serial8250_interrupt().
>> 
>> How are problems avoided when tty->low_latency is set?
>> 
>> Is tty->low_latency never allowed to be set for the 8250 driver?
>
> Several drivers certainly allow someone with superuser privileges to
> misconfigure them.

You don't need to be root to set the low_latency flag on a tty device.
Normal users are allowed to do that:

./linux/serial_core.h:361

#define UPF_USR_MASK  ((__force upf_t) (UPF_SPD_MASK|UPF_LOW_LATENCY))

> I've never worried about chasing all those down.

To me, it looks like anybody with permission for the serial port can
set the low latency flag and potentially cause a "scheduling while
atomic" crash.

I've gotten it to happen with other drivers that call
tty_flip_buffer_push() with tty->low_latency==1, but haven't been able
to make it happen with the 8250 driver -- I don't know why.  I've
verified that a normal user doing "setserial /dev/ttyS0 low_latency"
sets the low_latency flag, and it's 1 immediately before and after the
call to tty_flip_buffer_push().  Maybe I just haven't run it long
enough...

> It might actually be worth supporting a tty->low_latency mode which
> uses a tty layer private workqueue that is set to real time priority
> rather than the system work queue.

I don't understand why the low_latency flag even exists when setting
it apparently breaks most all of the serial drivers.

-- 
Grant Edwards               grant.b.edwards        Yow! You should all JUMP
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