locking/rwsem: Make handoff bit handling more consistent
There are some inconsistency in the way that the handoff bit is being handled in readers and writers that lead to a race condition. Firstly, when a queue head writer set the handoff bit, it will clear it when the writer is being killed or interrupted on its way out without acquiring the lock. That is not the case for a queue head reader. The handoff bit will simply be inherited by the next waiter. Secondly, in the out_nolock path of rwsem_down_read_slowpath(), both the waiter and handoff bits are cleared if the wait queue becomes empty. For rwsem_down_write_slowpath(), however, the handoff bit is not checked and cleared if the wait queue is empty. This can potentially make the handoff bit set with empty wait queue. Worse, the situation in rwsem_down_write_slowpath() relies on wstate, a variable set outside of the critical section containing the ->count manipulation, this leads to race condition where RWSEM_FLAG_HANDOFF can be double subtracted, corrupting ->count. To make the handoff bit handling more consistent and robust, extract out handoff bit clearing code into the new rwsem_del_waiter() helper function. Also, completely eradicate wstate; always evaluate everything inside the same critical section. The common function will only use atomic_long_andnot() to clear bits when the wait queue is empty to avoid possible race condition. If the first waiter with handoff bit set is killed or interrupted to exit the slowpath without acquiring the lock, the next waiter will inherit the handoff bit. While at it, simplify the trylock for loop in rwsem_down_write_slowpath() to make it easier to read. Fixes: 4f23dbc1 ("locking/rwsem: Implement lock handoff to prevent lock starvation") Reported-by:Zhenhua Ma <mazhenhua@xiaomi.com> Suggested-by: