Thanks, Mitch!
   That explains everything.

   Went reading pci_alloc_consistent()'s RTFM.
   That is exactly what I was missing in couple of my drivers.

Mitchell Blank Jr wrote:
> Ihar 'Philips' Filipau wrote:
> 
>>   Functions e1000_clean_{t,r}x_irq are very similar: both of them are 
>>checking descriptor flag updated by nic.
>>   Host CPU, obviously, to perform this check, will cache descriptor.
>>   If, say e1000_clean_rx_irq() will be called twice in short time 
>>range, I expect that it can miss change of the flag, since old flag may 
>>still sit in host CPU cache.
> 
> 
> Please see Documentation/DMA-mapping.txt; especially the part starting
> at "There are two types of DMA mappings..."  Ring buffers are allocated
> as "consistent" DMA memory.
> 
> For most architectures this will mean that the cache hardware snoops the
> PCI bus and automatically invalidates cache lines as they are written to.
> For architectures that can't do that then Linux will mark those memory
> regions uncacheable.
> 

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