linux-kernel - Re: [PATCH 2/2] kernfs: remove mutex in kernfs_dop

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Date:   Thu, 3 Dec 2020 14:35:54 +0800
From:   Fox Chen <foxhlchen@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] kernfs: remove mutex in kernfs_dop_revalidate

On Thu, Dec 3, 2020 at 2:26 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Wed, Dec 02, 2020 at 10:58:37PM +0800, Fox Chen wrote:
> > There is a big mutex in kernfs_dop_revalidate which slows down the
> > concurrent performance of kernfs.
> >
> > Since kernfs_dop_revalidate only does some checks, the lock is
> > largely unnecessary. Also, according to kernel filesystem locking
> > document:
> > https://www.kernel.org/doc/html/latest/filesystems/locking.html
> > locking is not in the protocal for d_revalidate operation.
> >
> > This patch remove this mutex from
> > kernfs_dop_revalidate, so kernfs_dop_revalidate
> > can run concurrently.
> >
> > Signed-off-by: Fox Chen <foxhlchen@...il.com>
> > ---
> >  fs/kernfs/dir.c | 9 +++------
> >  1 file changed, 3 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> > index 9aec80b9d7c6..c2267c93f546 100644
> > --- a/fs/kernfs/dir.c
> > +++ b/fs/kernfs/dir.c
> > @@ -26,7 +26,6 @@ static DEFINE_SPINLOCK(kernfs_idr_lock);    /* root->ino_idr */
> >
> >  static bool kernfs_active(struct kernfs_node *kn)
> >  {
> > -     lockdep_assert_held(&kernfs_mutex);
> >       return atomic_read(&kn->active) >= 0;
> >  }
> >
> > @@ -557,10 +556,9 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
> >
> >       /* Always perform fresh lookup for negatives */
> >       if (d_really_is_negative(dentry))
> > -             goto out_bad_unlocked;
> > +             goto out_bad;
> >
> >       kn = kernfs_dentry_node(dentry);
> > -     mutex_lock(&kernfs_mutex);
> >
> >       /* The kernfs node has been deactivated */
> >       if (!kernfs_active(kn))
> > @@ -579,11 +577,8 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
> >           kernfs_info(dentry->d_sb)->ns != kn->ns)
> >               goto out_bad;
> >
> > -     mutex_unlock(&kernfs_mutex);
> >       return 1;
> >  out_bad:
> > -     mutex_unlock(&kernfs_mutex);
> > -out_bad_unlocked:
> >       return 0;
> >  }
> >
> > @@ -650,6 +645,8 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
> >       kn->mode = mode;
> >       kn->flags = flags;
> >
> > +     rwlock_init(&kn->iattr_rwlock);
>
> Ah, now you initialize this, it should go into patch 1, right? :)
>
Yes, it's my fault. It should be in patch 1. Sorry.