Subject: Re: [RFC] [Patch 2/4] dccp: Lockless use of CCID blocks

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Sat, 20 Dec 2008 22:32:39 -0200

Em Sat, Dec 20, 2008 at 09:08:13AM +0100, Gerrit Renker escreveu:
> dccp: Lockless use of CCIDs
> 
> This updates the implementation to use only a single array whose size
> equals the number of configured CCIDs instead of 255.
> 
> Since the CCIDs are fixed array elements, synchronization is no longer
> needed.
> 
> Signed-off-by: Gerrit Renker <[email-masked]>
> ---
>  net/dccp/ccid.h |   10 ---
>  net/dccp/ccid.c |  166 +++++++++++---------------------------------------------
>  net/dccp/feat.c |    2 
>  3 files changed, 38 insertions(+), 140 deletions(-)
> 
> --- a/net/dccp/ccid.h
> +++ b/net/dccp/ccid.h
> @@ -19,14 +19,12 @@
>  #include <linux/list.h>
>  #include <linux/module.h>
>  
> -#define CCID_MAX 255
> -
>  struct tcp_info;
>  
>  /**
>   *  struct ccid_operations  -  Interface to Congestion-Control Infrastructure
>   *
> - *  @ccid_id: numerical CCID ID (up to %CCID_MAX, cf. table 5 in RFC 4340, 10.)
> + *  @ccid_id: numerical CCID ID (cf. table 5 in RFC 4340, 10.)
>   *  @ccid_ccmps: the CCMPS including network/transport headers (0 when disabled)
>   *  @ccid_name: alphabetical identifier string for @ccid_id
>   *  @ccid_hc_{r,t}x_slab: memory pool for the receiver/sender half-connection
> @@ -93,9 +91,6 @@ extern struct ccid_operations ccid2_ops;
>  extern struct ccid_operations ccid3_ops;
>  #endif
>  
> -extern int ccid_register(struct ccid_operations *ccid_ops);
> -extern int ccid_unregister(struct ccid_operations *ccid_ops);
> -
>  extern int ccids_register_builtins(void);
>  
>  struct ccid {
> @@ -113,8 +108,7 @@ extern int  ccid_get_builtin_ccids(u8 **
>  extern int  ccid_getsockopt_builtin_ccids(struct sock *sk, int len,
>  					  char __user *, int __user *);
>  
> -extern struct ccid *ccid_new(unsigned char id, struct sock *sk, int rx,
> -			     gfp_t gfp);
> +extern struct ccid *ccid_new(const u8 id, struct sock *sk, bool rx);
>  
>  static inline int ccid_get_current_rx_ccid(struct dccp_sock *dp)
>  {
> --- a/net/dccp/ccid.c
> +++ b/net/dccp/ccid.c
> @@ -13,8 +13,8 @@
>  
>  #include "ccid.h"
>  
> -static struct ccid_operations *builtin_ccids_ops[] = {
> -	&ccid2_ops,		/* CCID2 is supported by default */
> +static struct ccid_operations *ccids[] = {
> +	&ccid2_ops,	/* CCID-2 is supported by default */
>  #ifdef CONFIG_IP_DCCP_CCID3
>  	&ccid3_ops,
>  #endif
> @@ -27,49 +27,6 @@ static u8 builtin_ccids[] = {
>  #endif
>  };
>  
> -static struct ccid_operations *ccids[CCID_MAX];
> -#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
> -static atomic_t ccids_lockct = ATOMIC_INIT(0);
> -static DEFINE_SPINLOCK(ccids_lock);
> -
> -/*
> - * The strategy is: modifications ccids vector are short, do not sleep and
> - * veeery rare, but read access should be free of any exclusive locks.
> - */
> -static void ccids_write_lock(void)
> -{
> -	spin_lock(&ccids_lock);
> -	while (atomic_read(&ccids_lockct) != 0) {
> -		spin_unlock(&ccids_lock);
> -		yield();
> -		spin_lock(&ccids_lock);
> -	}
> -}
> -
> -static inline void ccids_write_unlock(void)
> -{
> -	spin_unlock(&ccids_lock);
> -}
> -
> -static inline void ccids_read_lock(void)
> -{
> -	atomic_inc(&ccids_lockct);
> -	smp_mb__after_atomic_inc();
> -	spin_unlock_wait(&ccids_lock);
> -}
> -
> -static inline void ccids_read_unlock(void)
> -{
> -	atomic_dec(&ccids_lockct);
> -}
> -
> -#else
> -#define ccids_write_lock() do { } while(0)
> -#define ccids_write_unlock() do { } while(0)
> -#define ccids_read_lock() do { } while(0)
> -#define ccids_read_unlock() do { } while(0)
> -#endif
> -
>  static struct kmem_cache *ccid_kmem_cache_create(int obj_size, const char *fmt,...)
>  {
>  	struct kmem_cache *slab;
> @@ -141,56 +98,33 @@ int ccid_getsockopt_builtin_ccids(struct
>  	return 0;
>  }
>  
> -int ccid_register(struct ccid_operations *ccid_ops)
> +static int ccid_register(struct ccid_operations *ccid_ops)
>  {
> -	int err = -ENOBUFS;
> -
>  	ccid_ops->ccid_hc_rx_slab =
>  			ccid_kmem_cache_create(ccid_ops->ccid_hc_rx_obj_size,
>  					       "ccid%u_hc_rx_sock",
>  					       ccid_ops->ccid_id);
>  	if (ccid_ops->ccid_hc_rx_slab == NULL)
> -		goto out;
> +		return -ENOBUFS;

You could have maintained the gotos, that way the patch would be
smaller...

>  
>  	ccid_ops->ccid_hc_tx_slab =
>  			ccid_kmem_cache_create(ccid_ops->ccid_hc_tx_obj_size,
>  					       "ccid%u_hc_tx_sock",
>  					       ccid_ops->ccid_id);
> -	if (ccid_ops->ccid_hc_tx_slab == NULL)
> -		goto out_free_rx_slab;
>  
> -	ccids_write_lock();

I.e. we would see that in the end what you did here was just to remove
the locking.

> -	err = -EEXIST;
> -	if (ccids[ccid_ops->ccid_id] == NULL) {
> -		ccids[ccid_ops->ccid_id] = ccid_ops;
> -		err = 0;
> +	if (ccid_ops->ccid_hc_tx_slab == NULL) {
> +		ccid_kmem_cache_destroy(ccid_ops->ccid_hc_rx_slab);
> +		ccid_ops->ccid_hc_rx_slab = NULL;
> +		return -ENOBUFS;
>  	}
> -	ccids_write_unlock();
> -	if (err != 0)
> -		goto out_free_tx_slab;
>  
>  	pr_info("CCID: Registered CCID %d (%s)\n",
>  		ccid_ops->ccid_id, ccid_ops->ccid_name);
> -out:
> -	return err;
> -out_free_tx_slab:
> -	ccid_kmem_cache_destroy(ccid_ops->ccid_hc_tx_slab);
> -	ccid_ops->ccid_hc_tx_slab = NULL;
> -	goto out;
> -out_free_rx_slab:
> -	ccid_kmem_cache_destroy(ccid_ops->ccid_hc_rx_slab);
> -	ccid_ops->ccid_hc_rx_slab = NULL;
> -	goto out;
> +	return 0;
>  }
>  
> -EXPORT_SYMBOL_GPL(ccid_register);
> -
> -int ccid_unregister(struct ccid_operations *ccid_ops)
> +static int ccid_unregister(struct ccid_operations *ccid_ops)
>  {
> -	ccids_write_lock();
> -	ccids[ccid_ops->ccid_id] = NULL;
> -	ccids_write_unlock();
> -
>  	ccid_kmem_cache_destroy(ccid_ops->ccid_hc_tx_slab);
>  	ccid_ops->ccid_hc_tx_slab = NULL;
>  	ccid_kmem_cache_destroy(ccid_ops->ccid_hc_rx_slab);

And "register/unregister" now don't make much sense since there is no
registration being done, just allocating/deallocating resources

> @@ -201,14 +135,12 @@ int ccid_unregister(struct ccid_operatio
>  	return 0;
>  }
>  
> -EXPORT_SYMBOL_GPL(ccid_unregister);
> -
>  int ccids_register_builtins(void)
>  {
>  	int i, err;
>  
> -	for (i = 0; i < ARRAY_SIZE(builtin_ccids_ops); i++) {
> -		err = ccid_register(builtin_ccids_ops[i]);
> +	for (i = 0; i < ARRAY_SIZE(ccids); i++) {
> +		err = ccid_register(ccids[i]);
>  		if (err)
>  			goto unwind_registrations;
>  	}
> @@ -217,15 +149,31 @@ int ccids_register_builtins(void)
>  
>  unwind_registrations:
>  	while(--i >= 0)
> -		ccid_unregister(builtin_ccids_ops[i]);
> +		ccid_unregister(ccids[i]);
>  	return err;
>  }
>  
> -static struct ccid *__ccid_new(struct ccid_operations *ccid_ops, struct sock *sk,
> -			       int rx, gfp_t gfp)
> +
> +static struct ccid_operations *ccid_find_by_id(const u8 id)
>  {
> -	struct ccid *ccid = kmem_cache_alloc(rx ? ccid_ops->ccid_hc_rx_slab :
> -						  ccid_ops->ccid_hc_tx_slab, gfp);
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(ccids); i++)
> +		if (ccids[i]->ccid_id == id)
> +			return ccids[i];
> +	return NULL;

Why the we searching? Can't we just do:

{
	if (id > ARRAY_SIZE(ccids) - 2)
		return NULL;
	return ccids[id - 2];
}

?

> +}
> +
> +struct ccid *ccid_new(const u8 id, struct sock *sk, bool rx)
> +{
> +	struct ccid_operations *ccid_ops = ccid_find_by_id(id);
> +	struct ccid *ccid;
> +
> +	if (ccid_ops == NULL)
> +		return NULL;
> +
> +	ccid = kmem_cache_alloc(rx ? ccid_ops->ccid_hc_rx_slab :
> +				     ccid_ops->ccid_hc_tx_slab, gfp_any());
>  	if (ccid == NULL)
>  		return NULL;
>  
> @@ -241,58 +189,14 @@ static struct ccid *__ccid_new(struct cc
>  		    ccid->ccid_ops->ccid_hc_tx_init(ccid, sk) != 0)
>  			goto out_free_ccid;
>  	}
> +
>  	return ccid;
> +
>  out_free_ccid:
>  	kmem_cache_free(rx ? ccid_ops->ccid_hc_rx_slab :
>  			     ccid_ops->ccid_hc_tx_slab, ccid);
>  	return NULL;
>  }
> -
> -static bool is_builtin_ccid(unsigned char id)
> -{
> -	int i;
> -	for (i = 0; i < ARRAY_SIZE(builtin_ccids); i++)
> -		if (id == builtin_ccids[i])
> -			return true;
> -	return false;
> -}
> -
> -struct ccid *ccid_new(unsigned char id, struct sock *sk, int rx, gfp_t gfp)
> -{
> -	struct ccid_operations *ccid_ops;
> -	struct ccid *ccid = NULL;
> -
> -	if (is_builtin_ccid(id)) {
> -		ccid_ops = ccids[id];
> -		BUG_ON(ccid_ops == NULL);
> -		return __ccid_new(ccid_ops, sk, rx, gfp);
> -	}
> -
> -	ccids_read_lock();
> -#ifdef CONFIG_MODULES
> -	if (ccids[id] == NULL) {
> -		/* We only try to load if in process context */
> -		ccids_read_unlock();
> -		if (gfp & GFP_ATOMIC)
> -			goto out;
> -		request_module("net-dccp-ccid-%d", id);
> -		ccids_read_lock();
> -	}
> -#endif
> -	ccid_ops = ccids[id];
> -	if (ccid_ops == NULL)
> -		goto out_unlock;
> -
> -	ccids_read_unlock();
> -
> -	ccid = __ccid_new(ccid_ops, sk, rx, gfp);
> -out:
> -	return ccid;
> -out_unlock:
> -	ccids_read_unlock();
> -	goto out;
> -}
> -
>  EXPORT_SYMBOL_GPL(ccid_new);
>  
>  static void ccid_delete(struct ccid *ccid, struct sock *sk, int rx)
> --- a/net/dccp/feat.c
> +++ b/net/dccp/feat.c
> @@ -34,7 +34,7 @@
>  static int dccp_hdlr_ccid(struct sock *sk, u64 ccid, bool rx)
>  {
>  	struct dccp_sock *dp = dccp_sk(sk);
> -	struct ccid *new_ccid = ccid_new(ccid, sk, rx, gfp_any());
> +	struct ccid *new_ccid = ccid_new(ccid, sk, rx);
>  
>  	if (new_ccid == NULL)
>  		return -ENOMEM;