linux-kernel - Re: [PATCH] mm: fix NULL checking in dma_pool

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Date:	Mon, 5 Nov 2012 13:26:51 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Xi Wang <xi.wang@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix NULL checking in dma_pool_create()

On Mon, 05 Nov 2012 15:50:32 -0500
Xi Wang <xi.wang@...il.com> wrote:

> On 11/5/12 3:37 PM, Andrew Morton wrote:
> > 
> > Well, the dma_pool_create() kerneldoc does not describe dev==NULL to be
> > acceptable usage and given the lack of oops reports, we can assume that
> > no code is calling this function with dev==NULL.
> > 
> > So I think we can just remove the code which handles dev==NULL?
> 
> Actually, a quick grep gives the following...
> 
> arch/arm/mach-s3c64xx/dma.c:731:	dma_pool = dma_pool_create("DMA-LLI", NULL, sizeof(struct pl080s_lli), 16, 0);
> drivers/usb/gadget/amd5536udc.c:3136:	dev->data_requests = dma_pool_create("data_requests", NULL,
> drivers/usb/gadget/amd5536udc.c:3148:	dev->stp_requests = dma_pool_create("setup requests", NULL,
> drivers/net/wan/ixp4xx_hss.c:973:		if (!(dma_pool = dma_pool_create(DRV_NAME, NULL,
> drivers/net/ethernet/xscale/ixp4xx_eth.c:1106:		if (!(dma_pool = dma_pool_create(DRV_NAME, NULL,
> 

OK, so it seems that those drivers have never been tested on a
CONFIG_NUMA kernel.  whee.

So we have a large amount of code here which ostensibly supports
dev==NULL but which has not been well tested.  Take a look at
dma_alloc_coherent(), dma_free_coherent() - are they safe?  Unobvious.

dmam_pool_destroy() will clearly cause an oops:

devres_destroy()
->devres_remove()
  ->spin_lock_irqsave(&dev->devres_lock, flags);

So what to do?

I'm thinking we should disallow dev==NULL.  We have a lot of code in
mm/dmapool.c which _attempts_ to support this case, but is largely
untested and obviously isn't working.  I don't think it's a good idea
to try to fix up and then support this case on behalf of a handful of
scruffy drivers.  It would be better to fix the drivers, then simplify
the core code.  drivers/usb/gadget/amd5536udc.c can probably use
dev->gadget.dev and drivers/net/wan/ixp4xx_hss.c can probably use
port->netdev->dev, etc.

So how about we add a WARN_ON_ONCE(dev == NULL), notify the driver maintainers
and later we can remove all that mm/dmapool.c code which is trying to
handle dev==NULL?

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