Re: [PATCH, RFC] xfs: remove i_iolock and use i_rwsem in the VFS inode instead

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Fri, Sep 09, 2016 at 10:33:06AM +0200, Christoph Hellwig wrote:
> On Thu, Sep 08, 2016 at 07:45:36AM +1000, Dave Chinner wrote:
> > It's a semaphore, not a mutex. Semaphore locking is independent of
> > task context, the lock follows the object it protects, not the task
> > that took the lock. i.e. Lockdep is wrong to assume the "owner" of a
> > rw_sem will not change between lock and unlock.
> 
> That's not the case - rw_semaphores had strict owner semanics for a
> long time (although I wish we could get rid of that for a different
> reason..).

Do tell; note however that due to the strict write owner, we can do
things like the optimistic spinning which improved writer->writer
performance significantly.

Also note that !owner locks create problems for RT.