Re: [PATCH] dmaengine: virt-dma: fix completion list manipulation

From: Lars-Peter Clausen
Date: Tue Mar 03 2015 - 06:21:38 EST

On 03/03/2015 11:27 AM, Robert Jarzmik wrote:
Lars-Peter Clausen <lars@xxxxxxxxxx> writes:

That will break all drivers which handle this currently correctly and remove the
descriptor from any list before calling vchan_cookie_complete.
Ah, well well I don't agree.

First, let's split the drivers which remove the descriptors and these which
don't :

These which remove the descriptor:

These which don't remove the descriptor:

All of those remove the descriptor from the list when they start the transfer.
Ah, I didn't see that.
Isn't the descriptor lost if a terminate_all(), relying on
vchan_get_all_descriptors() and vchan_dma_desc_free_list() is used ?

If the driver doesn't manually add it to the list of to be freed descriptors yes.

That settles the correctness I think, the correct behavior is to not remove the
descriptor and let it be done by vchan_cookie_complete().

Now for the remaining 2 drivers, we'll have :
- list_del(&vd->node) => vd becomes a singleton
- list_move_tail(&vd->node, &...desc_completed)
=> list_del(&vd->node) : nothing changes, it's a nop
=> list_add_tail(&vd->node, &...desc_completed)

And the behavior remains correct after the patch, only one "list_del()" is done
twice for nothing. Where do you see any breakage ?

Calling list_del() on a list item that is not on a list causes undefined
behavior, which can result in memory corruption, segfaults, etc...
Ah yes, you must be thinking about the "poisoning" after the __list_del() call,
I forgot about that.

Do you think amending all these drivers and changing their list_del() into
list_del_init() would at least prevent the "undefined behavior" ?


I still think that their use of virt-dma is incorrect, ie. that at one point in
time a virtual descriptor has to be on exactly one list of virt-dma (excepting
transient critical sections).

Well the drivers conform to the current expected behavior. It might be worth changing that, but you need to modify all the driver to conform to the new semantics, rather than just changing the API.

Requiring that the descriptor is always on one of the virt-dma list is to restrictive. Some DMA controllers are able to submit multiple descriptors at the same time, these typically have a separate list to manage to active descriptors.

- Lars

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