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

From: Robert Jarzmik
Date: Tue Mar 03 2015 - 05:27:16 EST

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:
>> dma-jz4740.c
>> fsl-edma.c
>> These which don't remove the descriptor:
>> amba-pl08x.c
>> edma.c
>> img-mdc-dma.c
>> k3dma.c
>> moxart-dma.c
>> omap-dma.c
>> qcom_bam_dma.c
>> s3c24xx-dma.c
>> sa11x0-dma.c
>> sun6i-dma.c
> 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 ?

>> 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).


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