serial: core: Allow processing sysrq at port unlock time
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:
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>