On Sat, Dec 09, 2017 at 11:25:19AM -0800, Tejun Heo wrote: > 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. > > Oleg, Peter, I'd really appreciate if you guys can go over the > reported breakages and the new implementation.
Great, yes that code seemed very suspicious when I looked at it; thanks for making it go away. I'll try and find a spot to stare at the patches.
