linux-kernel - Re: [PATCH 03/11] kernfs: add an API to get kernfs node from inode number

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Date:   Fri, 2 Jun 2017 16:36:55 -0700
From:   Shaohua Li <shli@...nel.org>
To:     Eduardo Valentin <eduval@...zon.com>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        tj@...nel.org, gregkh@...uxfoundation.org, hch@....de,
        axboe@...com, rostedt@...dmis.org, lizefan@...wei.com,
        Kernel-team@...com, Shaohua Li <shli@...com>
Subject: Re: [PATCH 03/11] kernfs: add an API to get kernfs node from inode
 number

On Fri, Jun 02, 2017 at 03:03:45PM -0700, Eduardo Valentin wrote:
> On Fri, Jun 02, 2017 at 02:53:56PM -0700, Shaohua Li wrote:
> > From: Shaohua Li <shli@...com>
> > 
> > Add an API to get kernfs node from inode number. We will need this to
> > implement exportfs operations.
> > 
> > To make the API lock free, kernfs node is freed in RCU context. And we
> > depend on kernfs_node count/ino number to filter stale kernfs nodes.
> > 
> > Signed-off-by: Shaohua Li <shli@...com>
> > ---
> >  fs/kernfs/dir.c             | 35 +++++++++++++++++++++++++++++++++++
> >  fs/kernfs/kernfs-internal.h |  2 ++
> >  fs/kernfs/mount.c           |  4 +++-
> >  3 files changed, 40 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> > index 8e8545a..4c86e4c 100644
> > --- a/fs/kernfs/dir.c
> > +++ b/fs/kernfs/dir.c
> > @@ -643,6 +643,7 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
> >  	kn->ino = ret;
> >  	kn->generation = atomic_inc_return(&root->next_generation);
> >  
> > +	/* set ino first. Above atomic_inc_return has a barrier */
> >  	atomic_set(&kn->count, 1);
> >  	atomic_set(&kn->active, KN_DEACTIVATED_BIAS);
> >  	RB_CLEAR_NODE(&kn->rb);
> > @@ -674,6 +675,40 @@ struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
> >  	return kn;
> >  }
> >  
> > +/*
> > + * kernfs_get_node_by_ino - get kernfs_node from inode number
> > + * @root: the kernfs root
> > + * @ino: inode number
> > + *
> > + * RETURNS:
> > + * NULL on failure. Return a kernfs node with reference counter incremented
> > + */
> 
> Is the above supposed to be a valid kernel doc entry?

what do you expect? The function name explains it very well actually.
 
> > +struct kernfs_node *kernfs_get_node_by_ino(struct kernfs_root *root,
> > +					   unsigned int ino)
> > +{
> > +	struct kernfs_node *kn;
> > +
> > +	rcu_read_lock();
> > +	kn = idr_find(&root->ino_idr, ino);
> > +	if (!kn)
> > +		goto out;
> > +	/* kernfs_put removes the ino after count is 0 */
> > +	if (!atomic_inc_not_zero(&kn->count)) {
> > +		kn = NULL;
> 
> Why do yo need to set kn to NULL?

I don't know what kind of explanation you expect. This is quite obvious
actually. If the count == 0, we don't increase the ref count, so we don't
decrease the ref count later (in kernfs_put).

> > +		goto out;
> > +	}
> > +	/* If this node is reused, __kernfs_new_node sets ino before count */
> > +	if (kn->ino != ino)
> > +		goto out;
> > +	rcu_read_unlock();
> > +
> > +	return kn;
> > +out:
> > +	rcu_read_unlock();
> > +	kernfs_put(kn);
> > +	return NULL;
> > +}
> > +
> >  /**
> >   *	kernfs_add_one - add kernfs_node to parent without warning
> >   *	@kn: kernfs_node to be added
> > diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
> > index 2d5144a..3534cfe 100644
> > --- a/fs/kernfs/kernfs-internal.h
> > +++ b/fs/kernfs/kernfs-internal.h
> > @@ -98,6 +98,8 @@ int kernfs_add_one(struct kernfs_node *kn);
> >  struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
> >  				    const char *name, umode_t mode,
> >  				    unsigned flags);
> > +struct kernfs_node *kernfs_get_node_by_ino(struct kernfs_root *root,
> > +					   unsigned int ino);
> >  
> >  /*
> >   * file.c
> > diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
> > index d5b149a..343dfeb 100644
> > --- a/fs/kernfs/mount.c
> > +++ b/fs/kernfs/mount.c
> > @@ -332,5 +332,7 @@ void __init kernfs_init(void)
> >  {
> >  	kernfs_node_cache = kmem_cache_create("kernfs_node_cache",
> >  					      sizeof(struct kernfs_node),
> > -					      0, SLAB_PANIC, NULL);
> > +					      0,
> > +					      SLAB_PANIC | SLAB_TYPESAFE_BY_RCU,
> > +					      NULL);
> >  }
> > -- 
> > 2.9.3
> > 
> > 
> 
> -- 
> All the best,
> Eduardo Valentin