Re: [PATCH 12/14] nfsd4: remove use of mutex for file_hashtable

[Benny Halevy <bhalevy@panasas.com>]

On Mar. 11, 2009, 2:27 +0200, "J. Bruce Fields" <bfields@fieldses.org> wrote:
> From: J. Bruce Fields <bfields@citi.umich.edu>
> 
> As part of reducing the scope of the client_mutex, and in order to
> remove the need for mutexes from the callback code (so that callbacks
> can be done as asynchronous rpc calls), move manipulations of the
> file_hashtable under the recall_lock.
> 
> Update the relevant comments while we're here.
> 
> Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
> Cc: Alexandros Batsakis <batsakis@netapp.com>
> ---
>  fs/nfsd/nfs4callback.c |    2 --
>  fs/nfsd/nfs4state.c    |   29 +++++++++++++++++------------
>  2 files changed, 17 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
> index 3fd7136..5dcd38e 100644
> --- a/fs/nfsd/nfs4callback.c
> +++ b/fs/nfsd/nfs4callback.c
> @@ -491,8 +491,6 @@ out_put_cred:
>  	 * or deleg_return.
>  	 */
>  	put_nfs4_client(clp);
> -	nfs4_lock_state();
>  	nfs4_put_delegation(dp);
> -	nfs4_unlock_state();
>  	return;
>  }
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 56e81f9..2f4cb9a 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -78,14 +78,18 @@ static struct nfs4_delegation * find_delegation_stateid(struct inode *ino, state
>  static char user_recovery_dirname[PATH_MAX] = "/var/lib/nfs/v4recovery";
>  static void nfs4_set_recdir(char *recdir);
>  
> -/* Locking:
> - *
> - * client_mutex:
> - * 	protects clientid_hashtbl[], clientstr_hashtbl[],
> - * 	unconfstr_hashtbl[], uncofid_hashtbl[].
> - */
> +/* Locking: */
> +
> +/* Currently used for almost all code touching nfsv4 state: */
>  static DEFINE_MUTEX(client_mutex);
>  
> +/*
> + * Currently used for the del_recall_lru and file hash table.  In an
> + * effort to decrease the scope of the client_mutex, this spinlock may
> + * eventually cover more:
> + */
> +static DEFINE_SPINLOCK(recall_lock);
> +
>  static struct kmem_cache *stateowner_slab = NULL;
>  static struct kmem_cache *file_slab = NULL;
>  static struct kmem_cache *stateid_slab = NULL;
> @@ -116,19 +120,15 @@ opaque_hashval(const void *ptr, int nbytes)
>  	return x;
>  }
>  
> -/*
> - * Delegation state
> - */
> -
> -/* recall_lock protects the del_recall_lru */
> -static DEFINE_SPINLOCK(recall_lock);
>  static struct list_head del_recall_lru;
>  
>  static void
>  free_nfs4_file(struct kref *kref)
>  {
>  	struct nfs4_file *fp = container_of(kref, struct nfs4_file, fi_ref);
> +	spin_lock(&recall_lock);
>  	list_del(&fp->fi_hash);
> +	spin_unlock(&recall_lock);
>  	iput(fp->fi_inode);
>  	kmem_cache_free(file_slab, fp);
>  }
> @@ -1004,7 +1004,9 @@ alloc_init_file(struct inode *ino)
>  		INIT_LIST_HEAD(&fp->fi_hash);
>  		INIT_LIST_HEAD(&fp->fi_stateids);
>  		INIT_LIST_HEAD(&fp->fi_delegations);
> +		spin_lock(&recall_lock);
>  		list_add(&fp->fi_hash, &file_hashtbl[hashval]);
> +		spin_unlock(&recall_lock);
>  		fp->fi_inode = igrab(ino);
>  		fp->fi_id = current_fileid++;
>  		fp->fi_had_conflict = false;
> @@ -1177,12 +1179,15 @@ find_file(struct inode *ino)
>  	unsigned int hashval = file_hashval(ino);
>  	struct nfs4_file *fp;
>  
> +	spin_lock(&recall_lock);
>  	list_for_each_entry(fp, &file_hashtbl[hashval], fi_hash) {
>  		if (fp->fi_inode == ino) {
> +			spin_unlock(&recall_lock);

Hmm, I'm afraid there's a race here since potentially
you could take the reference on the file after it has been freed.
find_file should call get_nfs4_file if and only if it's still hashed,
i.e. atomically with looking it up on the list.

On the same lines, the lock should be taken in put_nfs4_file
rather than in free_nfs4_file.

That being said, I'm not sure we're unhashing the file in the right
place... it's too late for me right now but my hunch is that open
should alloc_init the nfs4_file as done today and close should unhash
it (under the recall spinlock), and then put it.
put_nfs4_file can stay the same and free_nfs4_file should just do the
iput and kmem_cache_free.

The main difference is that find_file will fail finding the nfs4_file
struct after close.  (get_nfs4_file in find_file should still happen
under the lock though)

Benny

>  			get_nfs4_file(fp);
>  			return fp;
>  		}
>  	}
> +	spin_unlock(&recall_lock);
>  	return NULL;
>  }
>