From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by ozlabs.org (Postfix) with ESMTP id 7BB2C140111 for ; Thu, 3 Apr 2014 04:04:39 +1100 (EST) Date: Wed, 2 Apr 2014 22:05:19 +0530 From: Vinod Koul To: Hongbo Zhang Subject: Re: [PATCH 6/7] DMA: Freescale: use spin_lock_bh instead of spin_lock_irqsave Message-ID: <20140402163519.GU1976@intel.com> References: <1389851246-8564-1-git-send-email-hongbo.zhang@freescale.com> <1389851246-8564-7-git-send-email-hongbo.zhang@freescale.com> <1395817294.6569.2.camel@vkoul-udesk3> <533517C1.8080208@freescale.com> <20140329134528.GQ1976@intel.com> <5338EA57.1000509@freescale.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <5338EA57.1000509@freescale.com> Cc: Vinod Koul , linux-kernel@vger.kernel.org, scottwood@freescale.com, dmaengine@vger.kernel.org, dan.j.williams@intel.com, linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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@freescale.com wrote: > >>>>From: Hongbo Zhang > >>>> > >>>>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. -- ~Vinod > > If it is the problem, I would like to change dma_do_tasklet to > dma_tasklet to eliminate misleading. > > > --