linux-kernel - Re: [PATCH v2 02/31] cinergyT2-core: don't do DMA on stack

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Date:   Sat, 15 Oct 2016 22:54:49 +0200
From:   Johannes Stezenbach <js@...uxtv.org>
To:     Mauro Carvalho Chehab <mchehab@...pensource.com>
Cc:     Linux Media Mailing List <linux-media@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>,
        Patrick Boettcher <patrick.boettcher@...teo.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Michael Krufky <mkrufky@...uxtv.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jörg Otte <jrg.otte@...il.com>
Subject: Re: [PATCH v2 02/31] cinergyT2-core: don't do DMA on stack

On Tue, Oct 11, 2016 at 07:09:17AM -0300, Mauro Carvalho Chehab wrote:
> --- a/drivers/media/usb/dvb-usb/cinergyT2-core.c
> +++ b/drivers/media/usb/dvb-usb/cinergyT2-core.c
> @@ -41,6 +41,8 @@ DVB_DEFINE_MOD_OPT_ADAPTER_NR(adapter_nr);
>  
>  struct cinergyt2_state {
>  	u8 rc_counter;
> +	unsigned char data[64];
> +	struct mutex data_mutex;
>  };

Sometimes my thinking is slow but it just occured to me
that this creates a potential issue with cache line sharing.
On an architecture which manages cache coherence in software
(ARM, MIPS etc.) a write to e.g. rc_counter in this example
would dirty the cache line, and a later writeback from the
cache could overwrite parts of data[] which was received via DMA.
In contrast, if the DMA buffer is allocated seperately via
kmalloc it is guaranteed to be safe wrt cache line sharing.
(see bottom of Documentation/DMA-API-HOWTO.txt).

But of course DMA on stack also had the same issue
and no one ever noticed so it's apparently not critical...

Johannes