On 1/9/18 9:29 AM, Tejun Heo wrote: > 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. Applied for 4.16, and added a patch to silence that false positive srcu_idx for hctx_unlock() that got reported. This grows the request a bit, which sucks, but probably unavoidable. There's some room for improvement with a hole or two, however. -- Jens Axboe
