linux-kernel - Re: [RFC] dmaengine: pl330: fix a race condition in case of threaded irqs

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Message-ID: <CABb+yY3i5UaP2zzHmR3wrZMWpPEVPhas-3p-XDMDyus2nCqvAQ@mail.gmail.com>
Date:   Tue, 9 Jan 2018 13:59:45 +0530
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Qi Hou <qi.hou@...driver.com>
Cc:     Vinod Koul <vinod.koul@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        paul.gortmaker@...driver.com, xiao.zhang@...driver.com,
        Russell King <rmk+kernel@....linux.org.uk>,
        dmaengine@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] dmaengine: pl330: fix a race condition in case of threaded irqs

On Mon, Dec 25, 2017 at 7:50 AM, Qi Hou <qi.hou@...driver.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!