On Wed, May 11, 2016 at 08:03:46PM +0200, Michal Hocko wrote: > I still cannot say I would understand why the pending > RWSEM_WAITING_BIAS matters but I would probably need to look at the code > again with a clean head, __rwsem_wake is quite tricky...
Ah, you're asking why an unconditional __rwsem_wake(ANY) isn't enough? Because; if at that point there's nobody waiting, we're left with an empty list and WAITER_BIAS set. This in turn will make all fast paths fail. Look at rwsem_down_read_failed() for instance; if we enter that we'll unconditionally queue ourself, with nobody left to come wake us.
