linux-kernel - Re: [PATCH 6/7] DMA: Freescale: use spin_lock_bh instead of spin_lock

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Date:	Sat, 29 Mar 2014 19:15:28 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Hongbo Zhang <hongbo.zhang@...escale.com>
Cc:	Vinod Koul <vkoul@...radead.org>, dan.j.williams@...el.com,
	dmaengine@...r.kernel.org, scottwood@...escale.com,
	LeoLi@...escale.com, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] DMA: Freescale: use spin_lock_bh instead of
 spin_lock_irqsave

On Fri, Mar 28, 2014 at 02:33:37PM +0800, Hongbo Zhang wrote:
> 
> On 03/26/2014 03:01 PM, Vinod Koul wrote:
> >On Thu, 2014-01-16 at 13:47 +0800, hongbo.zhang@...escale.com wrote:
> >>From: Hongbo Zhang <hongbo.zhang@...escale.com>
> >>
> >>The usage of spin_lock_irqsave() is a stronger locking mechanism than is
> >>required throughout the driver. The minimum locking required should be used
> >>instead. Interrupts will be turned off and context will be saved, it is
> >>unnecessary to use irqsave.
> >>
> >>This patch changes all instances of spin_lock_irqsave() to spin_lock_bh(). All
> >>manipulation of protected fields is done using tasklet context or weaker, which
> >>makes spin_lock_bh() the correct choice.
> >>


> >>  /**
> >>@@ -1124,11 +1120,10 @@ static irqreturn_t fsldma_chan_irq(int irq, void *data)
> >>  static void dma_do_tasklet(unsigned long data)
> >>  {
> >>  	struct fsldma_chan *chan = (struct fsldma_chan *)data;
> >>-	unsigned long flags;
> >>  	chan_dbg(chan, "tasklet entry\n");
> >>-	spin_lock_irqsave(&chan->desc_lock, flags);
> >>+	spin_lock_bh(&chan->desc_lock);
> >okay here is the problem :(
> >
> >You moved to _bh variant. So if you grab the lock in rest of the code
> >and irq gets triggered then here we will be spinning to grab the lock.
> >So effectively you made right locking solution into deadlock situation!
> 
> If the rest code grabs lock by spin_lock_bh(), and if irq raised,
> the tasklet could not be executed because it has been disabled by
> the _bh variant function.
yes if you are accessing resources only in tasklet and rest of the code, then
_bh variant works well. The problem here is usage in irq handler

-- 
~Vinod
--
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