linux-kernel - Re: [PATCH] ipoctal: protect only the real critical section

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Date:	Thu, 26 Jun 2014 09:52:50 +0200
From:	Samuel Iglesias Gonsálvez 
	<siglesias@...lia.com>
To:	Federico Vaga <federico.vaga@...n.ch>
Cc:	jens.taprogge@...rogge.org, gregkh@...uxfoundation.org,
	industrypack-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipoctal: protect only the real critical section

On Thu, 2014-06-26 at 09:46 +0200, Federico Vaga wrote:
> In some conditions (echo or particular sequence of special
> characters), on buffer push, the tty layer calls the write operation
> while we are holding the spinlock. This means deadlock within the same
> process on kernels version < 3.12. It seems not a problem on recent
> kernel, but the patch still valid as locking optimization.
> 
> The protected variables by the spinlock are: xmit_buf, nb_bytes,
> pointer_read and pointer_write. So, this patch reduces the locked area
> in the IRQ handler only to these variables. Most of the code inside the
> locked area in the IRQ handler is not protected elsewhere; it means
> that it is not protected at all.
> 
> Signed-off-by: Federico Vaga <federico.vaga@...n.ch>
> ---
>  drivers/ipack/devices/ipoctal.c |    5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/ipack/devices/ipoctal.c b/drivers/ipack/devices/ipoctal.c
> index 141094e..69687f1 100644
> --- a/drivers/ipack/devices/ipoctal.c
> +++ b/drivers/ipack/devices/ipoctal.c
> @@ -177,19 +177,20 @@ static void ipoctal_irq_tx(struct ipoctal_channel *channel)
>  	if (channel->nb_bytes == 0)
>  		return;
>  
> +	spin_lock(&channel->lock);
>  	value = channel->tty_port.xmit_buf[*pointer_write];
>  	iowrite8(value, &channel->regs->w.thr);
>  	channel->stats.tx++;
>  	(*pointer_write)++;
>  	*pointer_write = *pointer_write % PAGE_SIZE;
>  	channel->nb_bytes--;
> +	spin_unlock(&channel->lock);
>  }
>  
>  static void ipoctal_irq_channel(struct ipoctal_channel *channel)
>  {
>  	u8 isr, sr;
>  
> -	spin_lock(&channel->lock);
>  	/* The HW is organized in pair of channels.  See which register we need
>  	 * to read from */
>  	isr = ioread8(&channel->block_regs->r.isr);
> @@ -213,8 +214,6 @@ static void ipoctal_irq_channel(struct ipoctal_channel *channel)
>  	/* TX of each character */
>  	if ((isr & channel->isr_tx_rdy_mask) && (sr & SR_TX_READY))
>  		ipoctal_irq_tx(channel);
> -
> -	spin_unlock(&channel->lock);
>  }
>  
>  static irqreturn_t ipoctal_irq_handler(void *arg)

Acked-by: Samuel Iglesias Gonsalvez <siglesias@...lia.com>

Sam

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