linux-kernel - Re: [PATCH V3 2/7] dmaengine/dw_dmac: Replace spin

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Date:	Thu, 28 Apr 2011 18:10:20 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Viresh Kumar <viresh.kumar@...com>
Cc:	vinod.koul@...el.com, dan.j.williams@...el.com,
	armando.visconti@...com, linux-kernel@...r.kernel.org,
	shiraz.hashim@...com, viresh.linux@...il.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH V3 2/7] dmaengine/dw_dmac: Replace spin_lock* with
	irqsave variants

On Wed, Apr 27, 2011 at 03:06:44PM +0530, Viresh Kumar wrote:
> @@ -407,6 +410,8 @@ EXPORT_SYMBOL(dw_dma_get_dst_addr);
>  static void dwc_handle_cyclic(struct dw_dma *dw, struct dw_dma_chan *dwc,
>  		u32 status_block, u32 status_err, u32 status_xfer)
>  {
> +	unsigned long flags;
> +
>  	if (status_block & dwc->mask) {
>  		void (*callback)(void *param);
>  		void *callback_param;
> @@ -418,9 +423,9 @@ static void dwc_handle_cyclic(struct dw_dma *dw, struct dw_dma_chan *dwc,
>  		callback = dwc->cdesc->period_callback;
>  		callback_param = dwc->cdesc->period_callback_param;
>  		if (callback) {
> -			spin_unlock(&dwc->lock);
> +			spin_unlock_irqrestore(&dwc->lock, flags);
>  			callback(callback_param);
> -			spin_lock(&dwc->lock);
> +			spin_lock_irqsave(&dwc->lock, flags);

I'm really not convinced that this is anywhere near correct.  I'm
surprised this doesn't spit out a compiler warning.

spin_unlock_irqrestore() reads the flags argument and puts it into
the PSR.  spin_lock_irqsave() reads the PSR, puts it into the flags
argument, sets the interrupt mask bit and writes back to the PSR.

So, if you do:

	unsigned long flags;

	spin_unlock_irqrestore(&dwc->lock, flags);
	...
	spin_lock_irqsave(&dwc->lock, flags);

you're going to end up corrupting the PSR.

In any case, releasing a spinlock temporarily within a called function
is _really_ not a nice thing to do.  It makes code review rather
difficult as called functions become non-atomic when called within
an atomic region.
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