[PATCH v2 0/6] block: block-status cache for data regions

[Max Reitz <mreitz@redhat.com>]

Hi,

See the cover letter from v1 for the general idea:

https://lists.nongnu.org/archive/html/qemu-block/2021-06/msg00843.html


The biggest change here in v2 is that instead of having a common CoMutex
protect the block-status cache, we’re using RCU now.  So to read from
the cache, or even to invalidate it, no lock is needed, only to update
it with new data.

Disclaimer: I have no experience with RCU in practice so far, neither in
qemu nor anywhere else.  So I hope I’ve used it correctly...


Differences to v1 in detail:
- Patch 2:
  - Moved BdrvBlockStatusCache.lock up to BDS, it is now the RCU writer
    lock
  - BDS.block_status_cache is now a pointer, so it can be replaced with
    RCU
  - Moved all cache access functionality into helper functions
    (bdrv_bsc_is_data(), bdrv_bsc_invalidate_range(), bdrv_bsc_fill())
    in block.c
  - Guard BSC accesses with RCU
    (BSC.valid is to be accessed atomically, which allows resetting it
    without taking an RCU write lock)
  - Check QLIST_EMPTY(&bs->children) not just when reading from the
    cache, but when filling it, too (so we don’t need an RCU update when
    it won’t make sense)
- Patch 3: Added
- Dropped the block/nbd patch (because it would make NBD query a larger
  range; the patch’s intent was to get more information for free, which
  this would not be)


git-backport-diff against v1:

Key:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch
[down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively

001/6:[----] [--] 'block: Drop BDS comment regarding bdrv_append()'
002/6:[0169] [FC] 'block: block-status cache for data regions'
003/6:[down] 'block: Clarify that @bytes is no limit on *pnum'
004/6:[----] [--] 'block/file-posix: Do not force-cap *pnum'
005/6:[----] [--] 'block/gluster: Do not force-cap *pnum'
006/6:[----] [--] 'block/iscsi: Do not force-cap *pnum'


Max Reitz (6):
  block: Drop BDS comment regarding bdrv_append()
  block: block-status cache for data regions
  block: Clarify that @bytes is no limit on *pnum
  block/file-posix: Do not force-cap *pnum
  block/gluster: Do not force-cap *pnum
  block/iscsi: Do not force-cap *pnum

 include/block/block_int.h | 54 +++++++++++++++++++++++--
 block.c                   | 84 +++++++++++++++++++++++++++++++++++++++
 block/file-posix.c        |  7 ++--
 block/gluster.c           |  7 ++--
 block/io.c                | 61 ++++++++++++++++++++++++++--
 block/iscsi.c             |  3 --
 6 files changed, 200 insertions(+), 16 deletions(-)

-- 
2.31.1