tree 60bda1e72adfea7a0a4fe3cdceacc78a3f0e8844
parent 3648888e90bb7fe6d0586ec177511e6678ee22c3
author Jeff Layton <jeff.layton@primarydata.com> 1428066244 -0400
committer Jeff Layton <jeff.layton@primarydata.com> 1428066244 -0400

locks: use cmpxchg to assign i_flctx pointer

During the v3.20/v4.0 cycle, I had originally had the code manage the
inode->i_flctx pointer using a compare-and-swap operation instead of the
i_lock.

Sasha Levin though hit a problem while testing with trinity that made me
believe that that wasn't safe. At the time, changing the code to protect
the i_flctx pointer seemed to fix the issue, but I now think that was
just coincidence.

The issue was likely the same race that Kirill Shutemov hit while
testing the pre-rc1 v4.0 kernel and that Linus spotted. Due to the way
that the spinlock was dropped in the middle of flock_lock_file, you
could end up with multiple flock locks for the same struct file on the
inode.

Reinstate the use of a CAS operation to assign this pointer since it's
likely to be more efficient and gets the i_lock completely out of the
file locking business.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>