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In-Reply-To: <1514168416-4285-1-git-send-email-qi.hou@windriver.com>
References: <1514168416-4285-1-git-send-email-qi.hou@windriver.com>
From: Jassi Brar <jassisinghbrar@gmail.com>
Date: Tue, 9 Jan 2018 13:59:45 +0530
Message-ID: <CABb+yY3i5UaP2zzHmR3wrZMWpPEVPhas-3p-XDMDyus2nCqvAQ@mail.gmail.com>
Subject: Re: [RFC] dmaengine: pl330: fix a race condition in case of threaded irqs
To: Qi Hou <qi.hou@windriver.com>
Cc: Vinod Koul <vinod.koul@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        paul.gortmaker@windriver.com, xiao.zhang@windriver.com,
        Russell King <rmk+kernel@arm.linux.org.uk>,
        dmaengine@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org

On Mon, Dec 25, 2017 at 7:50 AM, Qi Hou <qi.hou@windriver.com> wrote:
> I found this problem below, and I now understand why it happens, but I'm not
>  100% sure what is the best way to fix it.
>
> When booting up with "threadirqs" in command line, all irq handlers of the DMA
> controller pl330 will be threaded forcedly. These threads will race for the same
> list, pl330->req_done.
>
> Before the callback, the spinlock was released. And after it, the spinlock was
> taken. This opened an race window where another threaded irq handler could steal
> the spinlock and be permitted to delete entries of the list, pl330->req_done.
>
The locking has been recently modified beyond recognition, so I can't
tell why that part of code is the way it is.
The safest and cleanest solution seems to be to not drop and re-aquire the lock.

Cheers!