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

[Benny Halevy <bhalevy@panasas.com>]

On Mar. 18, 2009, 22:28 +0200, "J. Bruce Fields" <bfields@fieldses.org> wrote:
> On Fri, Mar 13, 2009 at 01:18:31PM -0400, bfields wrote:
>> On Thu, Mar 12, 2009 at 12:49:25PM +0200, Benny Halevy wrote:
>>> On Mar. 12, 2009, 2:05 +0200, "J. Bruce Fields" <bfields@fieldses.org> wrote:
>>>> On Wed, Mar 11, 2009 at 08:52:14PM +0200, Benny Halevy wrote:
>>>>>> @@ -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)
>>>> It's probably better for the nfs4_file to be visible longer, but either
>>>> is correct.
>>> I see. What matters is the stateids associated with the file.
>> Right.  So for now I'm taking your first suggestion.  Thanks again for
>> the review.
> 
> By the way, what I've got now is the following.--b.
> 
> commit 466c8f95fa8a70335002c2002f7a6c27b65e6e93
> Author: J. Bruce Fields <bfields@citi.umich.edu>
> Date:   Sun Feb 22 14:51:34 2009 -0800
> 
>     nfsd4: remove use of mutex for file_hashtable
>     
>     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>
>     Reviewed-by: Benny Halevy <bhalevy@panasas.com>
> 
> 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 fdcd7cf..a4bcfe0 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;
> @@ -116,33 +120,23 @@ 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);
> -	list_del(&fp->fi_hash);
> -	iput(fp->fi_inode);
> -	kmem_cache_free(file_slab, fp);
> -}
> -
>  static inline void
>  put_nfs4_file(struct nfs4_file *fi)
>  {
> -	kref_put(&fi->fi_ref, free_nfs4_file);

missing spin_lock(&recall_lock);?


> +	if (atomic_dec_and_lock(&fi->fi_ref, &recall_lock)) {
> +		list_del(&fi->fi_hash);
> +		spin_unlock(&recall_lock);
> +		iput(fi->fi_inode);
> +		kmem_cache_free(file_slab, fi);
> +	}

	} else {
		spin_unlock(&recall_lock);
	}

or am I missing something?

Benny

>  }
>  
>  static inline void
>  get_nfs4_file(struct nfs4_file *fi)
>  {
> -	kref_get(&fi->fi_ref);
> +	atomic_inc(&fi->fi_ref);
>  }
>  
>  static int num_delegations;
> @@ -1000,11 +994,13 @@ alloc_init_file(struct inode *ino)
>  
>  	fp = kmem_cache_alloc(file_slab, GFP_KERNEL);
>  	if (fp) {
> -		kref_init(&fp->fi_ref);
> +		atomic_set(&fp->fi_ref, 1);
>  		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 +1173,15 @@ 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) {
>  			get_nfs4_file(fp);
> +			spin_unlock(&recall_lock);
>  			return fp;
>  		}
>  	}
> +	spin_unlock(&recall_lock);
>  	return NULL;
>  }
>  
> diff --git a/include/linux/nfsd/state.h b/include/linux/nfsd/state.h
> index 503b6bb..a6e4a00 100644
> --- a/include/linux/nfsd/state.h
> +++ b/include/linux/nfsd/state.h
> @@ -214,7 +214,7 @@ struct nfs4_stateowner {
>  *      share_acces, share_deny on the file.
>  */
>  struct nfs4_file {
> -	struct kref		fi_ref;
> +	atomic_t		fi_ref;
>  	struct list_head        fi_hash;    /* hash by "struct inode *" */
>  	struct list_head        fi_stateids;
>  	struct list_head	fi_delegations;