[Cbe-oss-dev] spufs: kernel hangs by polling 'mfc' w/o proxy DMA request

[Kazunori Asayama]

I found that the kernel hangs if programs poll 'mfc' node of SPUFS
without proxy DMA requests. The reason why this problem occurs is
that:

  - If spufs_mfc_poll, which is the 'poll' operator of 'mfc', is
    called without proxy DMA requests, spufs_mfc_poll issues a proxy
    tag group query with query mask = 0 and query type = 2 (all):

	ctx->ops->set_mfc_query(ctx, ctx->tagwait, 2);

    The processor immediately raises a 'tag-group completion
    interrupt' corresponding to this query, because there is no
    outstanding proxy DMA at all.

  - The spufs_mfc_poll never (regardless of other conditions) returns
    POLLIN event when tagwait (a set of issued proxy DMA) is zero:

	if (tagstatus & ctx->tagwait)
		mask |= POLLIN | POLLRDNORM;

  - As a result of the above, spufs_mfc_poll endlessly issues proxy
    tag group queries with query mask = 0 and query type = 2, if once
    spufs_mfc_poll is called without proxy DMA request.

My questions are:

  - Why does spufs_mfc_poll issue queries with query type = 2 ? It
    seems strange that this condition (all) is different from
    spufs_mfc_read's one (any).

    I guess that this is just a workaround to implement the
    'SPE_TAG_ALL' behavior of spe_mfcio_tag_status_read without using
    incomplete fsync implementation.

    I remember that when we discussed how the behavior flags of
    spe_mfcio_tag_status_read should be implemented, we reached the
    conclusion as:

    * behaviors of operations on 'mfc' node:

      - blocking read on 'mfc'

      	blocks until at least one of the DMAs completes and then
      	returns all currently complete tag groups.

      - non-blocking read on 'mfc'

      	reads the current Prxy_TagStatus and masks it with tagwait.

      - fsync on 'mfc'

      	blocks until all DMAs (tagwait) are complete.

      - poll on 'mfc'

      	returns whenever any one of the DMAs completes.

    * implementations of libspe

      - spe_mfcio_tag_status_read with SPE_TAG_ALL: fsync then read.

      - spe_mfcio_tag_status_read with SPE_TAG_ANY: blocking read.

      - spe_mfcio_tag_status_read with SPE_TAG_IMMEDIATE: non-blocking read.

      - events: poll

  - If this my understanding is correct, is it OK that we will fix the
    fsync implementation then we will change the poll behavior so that
    it waits for 'any' condition?
    
    I think that there is no problem with doing so, because the
    libspe2's spe_mfcio_tag_status_read already has code to call fsync
    on 'mfc' for SPE_TAG_ALL and the libspe2 will still work correctly
    after this change.

  - How should the SPUFS behave with tagwait = 0?

    I think that followings are reasonable and useful for application
    programs:

    - poll: waits until new proxy DMAs will be issued and any of them
      	    will complete.

    - fsync: returns immediately.

Thanks,
--
(ASAYAMA Kazunori
  (asayama at sm.sony.co.jp))
t