linux-kernel - Re: [PATCH v2 0/6] atmel_serial: Cleanups, irq handler splitup & DMA

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Message-ID: <3efb10970712190840m39876762yb4d31c36723c3b8a@mail.gmail.com>
Date:	Wed, 19 Dec 2007 17:40:44 +0100
From:	"Remy Bohmer" <linux@...mer.net>
To:	"Haavard Skinnemoen" <hskinnemoen@...el.com>
Cc:	"Andrew Victor" <linux@...im.org.za>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	"Russell King" <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, kernel@...32linux.org
Subject: Re: [PATCH v2 0/6] atmel_serial: Cleanups, irq handler splitup & DMA

Hello Haavard,

> Could you try the patch below? It's a bit strange that you got an oops
> though...

It is not really strange... spinlocks are mutexes on preempt-rt, and
recursive mutex locking is not allowed, this is one differences with
the mainline spinlock.

But... I tried that patch, and it works a lot better, no oopses
anymore, but I noticed that I sometimes get an input overrun (ttyS0: 1
input overrun(s) ) during stress conditions.
This is something I did not notice before, maybe it was already there,
or has something changed in this area that it is now more sensitive
for this?

Kind Regards,

Remy

2007/12/19, Haavard Skinnemoen <hskinnemoen@...el.com>:
> On Wed, 19 Dec 2007 16:57:04 +0100
> "Remy Bohmer" <linux@...mer.net> wrote:
>
> > Hello Haavard,
> >
> > Sorry.. But I get an Oops on Preempt-RT with the latest set of
> > patches. I did not see it earlier today with the other set of patches.
>
> Hmm...from the backtrace, it looks like lock recursion -- port->lock is
> held for the whole duration of the tasklet, but we somehow end up in
> uart_start(), which grabs the lock again.
>
> Could you try the patch below? It's a bit strange that you got an oops
> though...
>
> Haavard
>
> diff --git a/drivers/serial/atmel_serial.c b/drivers/serial/atmel_serial.c
> index 7967054..948c643 100644
> --- a/drivers/serial/atmel_serial.c
> +++ b/drivers/serial/atmel_serial.c
> @@ -666,7 +666,13 @@ static void atmel_rx_from_ring(struct uart_port *port)
>                 uart_insert_char(port, status, ATMEL_US_OVRE, c.ch, flg);
>         }
>
> +       /*
> +        * Drop the lock here since it might end up calling
> +        * uart_start(), which takes the lock.
> +        */
> +       spin_unlock(&port->lock);
>         tty_flip_buffer_push(port->info->tty);
> +       spin_lock(&port->lock);
>  }
>
>  static void atmel_rx_from_dma(struct uart_port *port)
>
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