From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from tx2outboundpool.messaging.microsoft.com (tx2ehsobe001.messaging.microsoft.com [65.55.88.11]) (using TLSv1 with cipher AES128-SHA (128/128 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 50E23140146 for ; Thu, 3 Apr 2014 18:00:24 +1100 (EST) Message-ID: <533D06F2.7080702@freescale.com> Date: Thu, 3 Apr 2014 15:00:02 +0800 From: Hongbo Zhang MIME-Version: 1.0 To: Vinod Koul Subject: Re: [PATCH 6/7] DMA: Freescale: use spin_lock_bh instead of spin_lock_irqsave 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> <20140402163519.GU1976@intel.com> In-Reply-To: <20140402163519.GU1976@intel.com> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed 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 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@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. > OK, so I will send a v2 series with only updating 3/7 soon.