Changes from [v3]

- Rebased on top of for-4.16/block.

- Integrated Jens's hctx_[un]lock() factoring patch and refreshed the
  patches accordingly.

- Added comment explaining the use of hctx_lock() instead of
  rcu_read_lock() in completion path.

Changes from [v2]

- Possible extended looping around seqcount and u64_stat_sync fixed.

- Misplaced MQ_RQ_IDLE state setting fixed.

- RQF_MQ_TIMEOUT_EXPIRED added to prevent firing the same timeout
  multiple times.

- s/queue_rq_src/srcu/ patch added.

- Other misc changes.

Changes from [v1]

- BLK_EH_RESET_TIMER handling fixed.

- s/request->gstate_seqc/request->gstate_seq/

- READ_ONCE() added to blk_mq_rq_udpate_state().

- Removed left over blk_clear_rq_complete() invocation from

Currently, blk-mq timeout path synchronizes against the usual
issue/completion path using a complex scheme involving atomic
bitflags, REQ_ATOM_*, memory barriers and subtle memory coherence
rules.  Unfortunatley, it contains quite a few holes.

It's pretty easy to make blk_mq_check_expired() terminate a later
instance of a request.  If we induce 5 sec delay before
time_after_eq() test in blk_mq_check_expired(), shorten the timeout to
2s, and issue back-to-back large IOs, blk-mq starts timing out
requests spuriously pretty quickly.  Nothing actually timed out.  It
just made the call on a recycle instance of a request and then
terminated a later instance long after the original instance finished.
The scenario isn't theoretical either.

This patchset replaces the broken synchronization mechanism with a RCU
and generation number based one.  Please read the patch description of
the second path for more details.

This patchset contains the following eight patches.


and is available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git blk-mq-timeout-v4

diffstat follows.  Thanks.

 block/blk-core.c       |    2 
 block/blk-mq-debugfs.c |    4 
 block/blk-mq.c         |  340 ++++++++++++++++++++++++++++---------------------
 block/blk-mq.h         |   49 ++++++-
 block/blk-timeout.c    |   11 -
 block/blk.h            |    7 -
 include/linux/blk-mq.h |    3 
 include/linux/blkdev.h |   25 +++
 8 files changed, 283 insertions(+), 158 deletions(-)


[v3] http://lkml.kernel.org/r/20171216120726.517153-1...@kernel.org
[v2] http://lkml.kernel.org/r/20171010155441.753966-1...@kernel.org
[v1] http://lkml.kernel.org/r/20171209192525.982030-1...@kernel.org

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