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

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Date:	Mon, 31 Mar 2014 12:08:55 +0800
From:	Hongbo Zhang <hongbo.zhang@...escale.com>
To:	Vinod Koul <vinod.koul@...el.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 03/29/2014 09:45 PM, Vinod Koul wrote:
> 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
>

The name dma_do_tasklet may mislead, it is tasklet handler, not irq 
handler, not a trigger to load tasklet.
the irq handler is fsldma_chan_irq, and I don't use lock in it.

If it is the problem, I would like to change dma_do_tasklet to 
dma_tasklet to eliminate misleading.



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