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SubjectRe: [PATCH 08/11] vfs: Merge check_submounts_and_drop and d_invalidate
On Mon, Feb 24, 2014 at 04:01:29PM -0800, Eric W. Biederman wrote:
> Miklos Szeredi <miklos@szeredi.hu> writes:
>
> > On Sat, Feb 15, 2014 at 01:39:22PM -0800, Eric W. Biederman wrote:
> >>
> >> Now that d_invalidate is the only caller of check_submounts_and_drop,
> >> expand check_submounts_and_drop inline in d_invalidate.
> >>
> >> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> >> ---
> >> fs/dcache.c | 55 +++++++++++++++++++----------------------------
> >> include/linux/dcache.h | 1 -
> >> 2 files changed, 22 insertions(+), 34 deletions(-)
> >>
> >> diff --git a/fs/dcache.c b/fs/dcache.c
> >> index 27585b1dd6f1..5b41205cbf33 100644
> >> --- a/fs/dcache.c
> >> +++ b/fs/dcache.c
>
> >> -int check_submounts_and_drop(struct dentry *dentry)
> >> +int d_invalidate(struct dentry *dentry)
> >> {
> >> int ret = 0;
> >>
> >> + /*
> >> + * If it's already been dropped, return OK.
> >> + */
> >> + spin_lock(&dentry->d_lock);
> >> + if (d_unhashed(dentry)) {
> >> + spin_unlock(&dentry->d_lock);
> >> + return 0;
> >> + }
> >> + spin_unlock(&dentry->d_lock);
> >> +
> >> /* Negative dentries can be dropped without further checks */
> >> if (!dentry->d_inode) {
> >> d_drop(dentry);
> >
> >
> > You can optimize this by including the negative check within the above d_locked
> > region and calling __d_drop() instead.
>
> For this patch just moving the code and not changing it is the corret
> thing to do because it helps with review and understanding the code.
>
> There are two ways I could see going with optimizing the preamble.
> Simply dropping the d_lock from around the d_unhashed test as a pointer
> dereference should be atomic, and the test is racy against
> d_materialise_unique.

Could you explain? What's the race, and what are the consequences?

--b.

> (We don't always hold the parent
> directories inode mutex when d_invalidate is called). So the d_lock
> buys us very little. Alternatively we could move the work into the
> d_walk callbacks.
>
> That kind of optimization deserves it's own patch that can be reviewed
> independently.
>
> Eric
> --
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