From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga11.intel.com (mga11.intel.com [192.55.52.93]) by ozlabs.org (Postfix) with ESMTP id ACD2B140081 for ; Sun, 30 Mar 2014 00:52:36 +1100 (EST) Date: Sat, 29 Mar 2014 19:15:28 +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: <20140329134528.GQ1976@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <533517C1.8080208@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 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 -- ~Vinod