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

[Dave Chinner <david@xxxxxxxxxxxxx>]

On Mon, Sep 05, 2016 at 05:15:29PM +0200, Christoph Hellwig wrote:
> Hi Dave,
> 
> I looked into killing the mrlock and ran into an unexpected problem.
> 
> Currently mr_writer tracks that there is someone holding a write lock,
> lockdep on the other hand checks if the calling thread has that lock.
> 
> While that generally is the right semantic, our hack to offload
> btree splits to a work item offends lockdep.  E.g. this callstack
> now asserts:

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.

> While it previously did fine.  I fear there might be other locking
> asserts in the code called from that work item as well.

Probably.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx