From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ch1outboundpool.messaging.microsoft.com (ch1ehsobe003.messaging.microsoft.com [216.32.181.183]) (using TLSv1 with cipher AES128-SHA (128/128 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 47BC11401F0 for ; Mon, 31 Mar 2014 15:09:15 +1100 (EST) Message-ID: <5338EA57.1000509@freescale.com> Date: Mon, 31 Mar 2014 12:08:55 +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> In-Reply-To: <20140329134528.GQ1976@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 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. If it is the problem, I would like to change dma_do_tasklet to dma_tasklet to eliminate misleading.