Re: performance delta after VFS i_mutex=>i_rwsem conversion

[Ingo Molnar <mingo@kernel.org>]

* Waiman Long <waiman.long@hpe.com> wrote:

> I do have a patchset that allow us to more accurately determine the state of
> the lock owner.
> 
> locking/rwsem: Add reader-owned state to the owner field
> http://www.spinics.net/lists/kernel/msg2258572.html
> 
> That should eliminate the performance gap between mutex and rwsem wrt
> spinning when only writers are present. I am hoping that that patchset can
> be queued for 4.8.

Yeah, so I actually had this series merged for testing last week, but a 
complication with a prereq patch made me unmerge it. But I have no fundamental 
objections, at all.

I also agree with Linus's general observation that we want to make 
down_write()/up_write() match mutex performance characteristics.

I think kernel developers should fundamentally be able to switch between 
mutex_lock()/unlock() and down_write()/up_write() and back, without noticing
any high level behavioral changes.

Any 'reader/writer mixing' artifacts are secondary concerns and we'll sort them 
out as they happen.

Thanks,

	Ingo