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Subject[PATCHSET v4] blk-mq: reimplement timeout handling
Date
Hello,

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
blk_mq_rq_timed_out().

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.

0001-blk-mq-move-hctx-lock-unlock-into-a-helper.patch
0002-blk-mq-protect-completion-path-with-RCU.patch
0003-blk-mq-replace-timeout-synchronization-with-a-RCU-an.patch
0004-blk-mq-use-blk_mq_rq_state-instead-of-testing-REQ_AT.patch
0005-blk-mq-make-blk_abort_request-trigger-timeout-path.patch
0006-blk-mq-remove-REQ_ATOM_COMPLETE-usages-from-blk-mq.patch
0007-blk-mq-remove-REQ_ATOM_STARTED.patch
0008-blk-mq-rename-blk_mq_hw_ctx-queue_rq_srcu-to-srcu.patch

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(-)

--
tejun

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

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