Hi,
On Thu, 2011-08-18 at 16:38 -0400, David Teigland wrote:
> When i_flock contains only flocks, the posix unlock is
> extraneous. On gfs2, ocfs2, and possibly others, the
> posix unlock can be costly, and something to avoid if
> possible.
>
> As the comment implies, there will be races here. The
> question is, do those races become harmful with this
> new loop?
>
The problem is that you don't hold the right lock (file_lock_lock) at
this point, so its ok to test the i_flock pointer for being NULL, but
its not ok to dereference it.
Since this check is made for every file that is closed, taking a global
spinlock in this code path is frowned upon from a performance PoV.
> Signed-off-by: David Teigland <teigland@xxxxxxxxxx>
> ---
> fs/locks.c | 14 +++++++++++++-
> 1 files changed, 13 insertions(+), 1 deletions(-)
>
> diff --git a/fs/locks.c b/fs/locks.c
> index 703f545..4507401 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -1982,15 +1982,27 @@ out:
> void locks_remove_posix(struct file *filp, fl_owner_t owner)
> {
> struct file_lock lock;
> + struct file_lock **before;
> + struct inode *inode;
>
> /*
> * If there are no locks held on this file, we don't need to call
> * posix_lock_file(). Another process could be setting a lock on this
> * file at the same time, but we wouldn't remove that lock anyway.
> */
> - if (!filp->f_path.dentry->d_inode->i_flock)
> + inode = filp->f_path.dentry->d_inode;
> +
> + if (!inode->i_flock)
> return;
>
So if you take the lock at this point, and drop it after the loop, that
should resolve the issue without incurring the extra overhead on every
single close,
Steve.
> + for_each_lock(inode, before) {
> + struct file_lock *fl = *before;
> + if (IS_POSIX(fl))
> + goto do_unlock;
> + }
> + return;
> +
> +do_unlock:
> lock.fl_type = F_UNLCK;
> lock.fl_flags = FL_POSIX | FL_CLOSE;
> lock.fl_start = 0;
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