Re: [PATCH v7 0/4] xfs: Remove wrappers for some semaphores

[Christoph Hellwig <hch@infradead.org>]

On Mon, Mar 23, 2020 at 09:33:42AM -0700, Darrick J. Wong wrote:
> lockdep tracks the rwsem's lock state /and/ which process actually
> holds the rwsem.  This ownership doesn't transfer from 28177 to 27081,
> so when the kworker asks lockdep if it holds ILOCK_EXCL, lockdep says
> no, because 27081 doesn't own the lock, 28177 does.  Kaboom.

> The old mrlock_t had that 'int mr_writer' field which didn't care about
> lock ownership and so isilocked would return true and so the assert was
> happy.
> 
> So now comes the fun part -- the old isilocked code had a glaring hole
> in which it would return true if *anyone* held the lock, even if the
> owner is some other unrelated thread.  That's probably good enough for
> most of the fstests because we generally only run one thread at a time,
> and developers will probably notice. :)
> 
> However, with your series applied, the isilocked function becomes much
> more powerful when lockdep is active because now we can test that the
> lock is held *by the caller*, which closes that hole.
> 
> Unfortunately, it also trips over this bmap split case, so if there's a
> way to solve that problem we'd better find it quickly.  Unfortunately, I
> don't know of a way to gift a lock to another thread temporarily...
> 
> Thoughts?

lock_release() in the donor, lock_acquire in the worker context, and
vice versa on return.  We already do that dance indirectly with
xfs_setfilesize_trans_alloc and friends.