Re: [PATCH v5] locking/rwsem: Make handoff bit handling more consistent

[Peter Zijlstra]

On Tue, Nov 16, 2021 at 10:52:42AM +0800, Aiqun(Maria) Yu wrote:
> On 11/16/2021 9:29 AM, Waiman Long wrote:
> > There are some inconsistency in the way that the handoff bit is being
> > handled in readers and writers.
> > 
> > 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.
> > 
> > To make the handoff bit handling more consistent and robust, extract
> > out handoff bit clearing code into the new rwsem_del_waiter() helper
> > function.  The common function will only use atomic_long_andnot() to
> > clear bits when the wait queue is empty to avoid possible race condition.
> we do have race condition needed to be fixed with this change.

Indeed, let me edit the changelog to reflect that. Also, I think, it
needs a Reported-by:.

> > 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: 4f23dbc1e657 ("locking/rwsem: Implement lock handoff to prevent lock starvation")
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>