Hi Daniel,
On Fri, May 4, 2018 at 6:30 PM, Daniel Wagner <wagi@xxxxxxxxx> wrote:
> From: Daniel Wagner <daniel.wagner@xxxxxxxxxxx>
>
> Commit 40f70c03e33a ("serial: sh-sci: add locking to console write
> function to avoid SMP lockup") copied the strategy to avoid locking
> problems in conjuncture with the console from the UART8250
> driver. Instead using directly spin_{try}lock_irqsave(),
> local_irq_save() followed by spin_{try}lock() was used. While this is
> correct on mainline, for -rt it is a problem. spin_{try}lock() will
> check if it is running in a valid context. Since the local_irq_save()
> has already been executed, the context has changed and
> spin_{try}lock() will complain. The reason why spin_{try}lock()
> complains is that on -rt the spin locks are turned into mutexes and
> therefore can sleep. Sleeping with interrupts disabled is not valid.
[...]
> ---
>
> changes since v1:
> - Ported to current mainline (initial version was against v4.4.y)
> - Left local_irq_save() in place when spinlocks are not used as suggested
> by Geert.
Thanks for the update!
> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -2890,16 +2890,16 @@ static void serial_console_write(struct console *co, const char *s,
> unsigned long flags;
> int locked = 1;
>
> - local_irq_save(flags);
> #if defined(SUPPORT_SYSRQ)
> - if (port->sysrq)
> + if (port->sysrq) {
> locked = 0;
> - else
> + local_irq_save(flags);
> + } else
> #endif
> if (oops_in_progress)
> - locked = spin_trylock(&port->lock);
> + locked = spin_trylock_irqsave(&port->lock, flags);
If the spinlock could not be taken, interrupts are re-enabled:
include/linux/spinlock.h:
#define raw_spin_trylock_irqsave(lock, flags) \
({ \
local_irq_save(flags); \
raw_spin_trylock(lock) ? \
1 : ({ local_irq_restore(flags); 0; }); \
})
hence I think you need to check for this and disable interrupts again:
if (!locked)
local_irq_save(flags);
> else
> - spin_lock(&port->lock);
> + spin_lock_irqsave(&port->lock, flags);
>
> /* first save SCSCR then disable interrupts, keep clock source */
> ctrl = serial_port_in(port, SCSCR);
> @@ -2919,8 +2919,9 @@ static void serial_console_write(struct console *co, const char *s,
> serial_port_out(port, SCSCR, ctrl);
>
> if (locked)
> - spin_unlock(&port->lock);
> - local_irq_restore(flags);
> + spin_unlock_irqrestore(&port->lock, flags);
> + else
> + local_irq_restore(flags);
> }
>
> static int serial_console_setup(struct console *co, char *options)
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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