Re: [PATCH 6/7] DMA: Freescale: use spin_lock_bh instead of spin_lock_irqsave

[Hongbo Zhang]

On 04/03/2014 12:35 AM, Vinod Koul wrote:
> On Mon, Mar 31, 2014 at 12:08:55PM +0800, Hongbo Zhang wrote:
> > 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@xxxxxxxxxxxxx wrote:
> > > > > > From: Hongbo Zhang <hongbo.zhang@xxxxxxxxxxxxx>
> > > > > >
> > > > > > 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.
> sorry my bad, i misread this as code in fsldma_chan_irq() insteadof
> dma_do_tasklet(). In that case patch is doing the right thing.

OK, so I will send a v2 series with only updating 3/7 soon.




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