linux-kernel - Re: [PATCH -mm v4 6/9] atmel_serial: Split the interrupt handler

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Date:	Wed, 30 Jan 2008 10:41:13 +0100
From:	Haavard Skinnemoen <hskinnemoen@...el.com>
To:	michael <trimarchi@...dalf.sssup.it>
Cc:	Remy Bohmer <linux@...mer.net>, fabio@...dalf.sssup.it,
	Andrew Victor <linux@...im.org.za>,
	Chip Coldwell <coldwell@...hat.com>,
	Marc Pignat <marc.pignat@...s.ch>,
	David Brownell <david-b@...bell.net>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH -mm v4 6/9] atmel_serial: Split the interrupt handler

On Wed, 30 Jan 2008 00:12:23 +0100
michael <trimarchi@...dalf.sssup.it> wrote:

> I'm testing this patch on an at91sam9260 on 2.6.24-rt. I'm using this
> patch with the tclib support for hrtimer and the clocksource pit_clk.
> These are the results:
> 
> - Voluntary Kernel Preemption the system (crash)
> - Preemptible  Kernel (crash)

Ouch. I'm assuming this is with DMA disabled?

> /*
>  * 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);
>  */
> The same code with this comments out runs

Now, _that_ is strange. I can't see anything that needs protection
across that call; in fact, I think we can lock a lot less than what we
currently do.

> Complete Preemption (Real-Time) ok but the serials is just unusable due
> to too many overruns (just using lrz)

Is it worse than before? IIRC Remy mentioned something about
IRQF_NODELAY being the reason for moving all this code to softirq
context in the first place; does the interrupt handler run in hardirq
context?

> The system is stable and doesn't crash reverting this patch.
> I don't test with the thread hardirqs active.

Ok.

> >> +	ret = -ENOMEM;
> >> +	data = kmalloc(ATMEL_SERIAL_RINGSIZE, GFP_KERNEL);
> >> +	if (!data)
> >> +		goto err_alloc_ring;
> >> +	port->rx_ring.buf = data;
> >> +
> >>  	ret = uart_add_one_port(&atmel_uart, &port->uart);
> >>     
> Is the kmalloc correct?
> maybe:
> data = kmalloc(ATMEL_SERIAL_RINGSIZE * sizeof(struct atmel_uart_char), 
> GFP_KERNEL);

I think you're right. Can you change it and see if it helps?

I guess I didn't test it thoroughly enough with DMA
disabled...slub_debug ought to catch such things, but not until we
receive enough data to actually overflow the buffer.

> >> @@ -1033,6 +1165,9 @@ static int __devexit atmel_serial_remove(struct platform_device *pdev)
> >>  
> >>  	ret = uart_remove_one_port(&atmel_uart, port);
> >>  
> >> +	tasklet_kill(&atmel_port->tasklet);
> >> +	kfree(atmel_port->rx_ring.buf);
> >> +
> >>     
> Why the tasklet_kill is not done in atmel_shutdown?

Why should it be? If it should, we must move the call to tasklet_init
into atmel_startup too, and I don't really see the point.

Haavard
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