Hello, Changes from [v4]
- Comments added. Patch description updated. 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-v5 diffstat follows. Thanks. block/blk-core.c | 2 block/blk-mq-debugfs.c | 4 block/blk-mq.c | 346 +++++++++++++++++++++++++++++-------------------- block/blk-mq.h | 49 ++++++ block/blk-timeout.c | 16 +- block/blk.h | 7 include/linux/blk-mq.h | 3 include/linux/blkdev.h | 25 +++ 8 files changed, 294 insertions(+), 158 deletions(-) -- tejun [v4] http://lkml.kernel.org/r/20180108191542.379478-1...@kernel.org [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 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
