Re: [PATCH v3] dmaengine: idxd: fix submission race window

[Vinod Koul <vkoul@kernel.org>]

On 29-06-21, 09:11, Dave Jiang wrote:
> Konstantin observed that when descriptors are submitted, the descriptor is
> added to the pending list after the submission. This creates a race window
> with the slight possibility that the descriptor can complete before it
> gets added to the pending list and this window would cause the completion
> handler to miss processing the descriptor.
> 
> To address the issue, the addition of the descriptor to the pending list
> must be done before it gets submitted to the hardware. However, submitting
> to swq with ENQCMDS instruction can cause a failure with the condition of
> either wq is full or wq is not "active".
> 
> With the descriptor allocation being the gate to the wq capacity, it is not
> possible to hit a retry with ENQCMDS submission to the swq. The only
> possible failure can happen is when wq is no longer "active" due to hw
> error and therefore we are moving towards taking down the portal. Given
> this is a rare condition and there's no longer concern over I/O
> performance, the driver can walk the completion lists in order to retrieve
> and abort the descriptor.
> 
> The error path will set the descriptor to aborted status. It will take the
> work list lock to prevent further processing of worklist. It will do a
> delete_all on the pending llist to retrieve all descriptors on the pending
> llist. The delete_all action does not require a lock. It will walk through
> the acquired llist to find the aborted descriptor while add all remaining
> descriptors to the work list since it holds the lock. If it does not find
> the aborted descriptor on the llist, it will walk through the work
> list. And if it still does not find the descriptor, then it means the
> interrupt handler has removed the desc from the llist but is pending on
> the work list lock and will process it once the error path releases the
> lock.

This fails to apply on fixes branch, pls rebase

-- 
~Vinod