dmaengine Archive on lore.kernel.org
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From: Vinod Koul <vkoul@kernel.org>
To: Logan Gunthorpe <logang@deltatee.com>
Cc: linux-kernel@vger.kernel.org, dmaengine@vger.kernel.org,
	Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH 1/5] dmaengine: Store module owner in dma_device struct
Date: Thu, 14 Nov 2019 10:25:55 +0530
Message-ID: <20191114045555.GJ952516@vkoul-mobl> (raw)
In-Reply-To: <5ca7ef5d-dda7-e36c-1d40-ef67612d2ac4@deltatee.com>

On 12-11-19, 09:45, Logan Gunthorpe wrote:
> 
> 
> On 2019-11-11 10:56 p.m., Vinod Koul wrote:
> > On 11-11-19, 09:50, Logan Gunthorpe wrote:
> >>
> >>
> >> On 2019-11-09 10:18 a.m., Vinod Koul wrote:
> >>> Hi Logan,
> >>>
> >>> Sorry for delay in reply!
> >>>
> >>> On 22-10-19, 15:46, Logan Gunthorpe wrote:
> >>>> dma_chan_to_owner() dereferences the driver from the struct device to
> >>>> obtain the owner and call module_[get|put](). However, if the backing
> >>>> device is unbound before the dma_device is unregistered, the driver
> >>>> will be cleared and this will cause a NULL pointer dereference.
> >>>
> >>> Have you been able to repro this? If so how..?
> >>>
> >>> The expectation is that the driver shall unregister before removed.
> >>
> >> Yes, with my new driver, if I do a PCI unbind (which unregisters) while
> >> the DMA engine is in use, it panics. The point is the underlying driver
> >> can go away before the channel is removed.
> > 
> > and in your driver remove you do not unregister? When unbind is invoked
> > the driver remove is invoked by core and you should unregister whatever
> > you have registered in your probe!
> >
> > Said that, if someone is using the dmaengine at that point of time, it
> > is not a nice thing to do and can cause issues, but on idle it should
> > just work!
> 
> But that's the problem. We can't expect our users to be "nice" and not
> unbind when the driver is in use. Killing the kernel if the user
> unexpectedly unbinds is not acceptable.

And that is why we review the code and ensure this does not happen and
behaviour is as expected

> >> I suspect this is less of an issue for most devices as they wouldn't
> >> normally be unbound while in use (for example there's really no reason
> >> to ever unbind IOAT seeing it's built into the system). Though, the fact
> >> is, the user could unbind these devices at anytime and we don't want to
> >> panic if they do.
> > 
> > There are many drivers which do modules so yes I am expecting unbind and
> > even a bind following that to work
> 
> Except they will panic if they unbind while in use, so that's a
> questionable definition of "work".

dmaengine core has module reference so while they are being used they
won't be removed (unless I complete misread the driver core behaviour)

-- 
~Vinod

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-22 21:46 [PATCH 0/5] PLX Switch DMA Engine Driver Logan Gunthorpe
2019-10-22 21:46 ` [PATCH 1/5] dmaengine: Store module owner in dma_device struct Logan Gunthorpe
2019-11-09 17:18   ` Vinod Koul
2019-11-11 16:50     ` Logan Gunthorpe
2019-11-12  5:56       ` Vinod Koul
2019-11-12 16:45         ` Logan Gunthorpe
2019-11-14  4:55           ` Vinod Koul [this message]
2019-11-14 17:03             ` Logan Gunthorpe
2019-11-22  5:20               ` Vinod Koul
2019-11-22 16:53                 ` Dave Jiang
2019-11-22 20:50                   ` Dan Williams
2019-11-22 20:56                     ` Logan Gunthorpe
2019-11-22 21:01                       ` Dan Williams
2019-11-22 21:42                         ` Dave Jiang
2019-12-10  9:53                           ` Vinod Koul
2019-12-10 17:39                             ` Logan Gunthorpe
2019-10-22 21:46 ` [PATCH 2/5] dmaengine: Call module_put() after device_free_chan_resources() Logan Gunthorpe
2019-10-22 21:46 ` [PATCH 3/5] dmaengine: plx-dma: Introduce PLX DMA engine PCI driver skeleton Logan Gunthorpe
2019-11-09 17:35   ` Vinod Koul
2019-11-11 17:50     ` Logan Gunthorpe
2019-11-12  6:09       ` Vinod Koul
2019-11-12 17:22         ` Logan Gunthorpe
2019-10-22 21:46 ` [PATCH 4/5] dmaengine: plx-dma: Implement hardware initialization and cleanup Logan Gunthorpe
2019-10-22 21:46 ` [PATCH 5/5] dmaengine: plx-dma: Implement descriptor submission Logan Gunthorpe
2019-11-09 17:40   ` Vinod Koul
2019-11-11 18:11     ` Logan Gunthorpe

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