Re: [RFC, PATCH] kernel/rcu: add kfree_rcu

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Hi, Manfred Spraul,

Thanks you for re-propose kfree_rcu().
I NAK. this patch.

Comparing with my patch(V1), your patch has several improvements.
But core idea is the same as my patch.
This idea(share offset with function pointer) has some disadvantages.
And I had implemented V2 and the patches for using kfree_rcu().
The V2 patch has not tested enough, but It will be done in one month.

Thanks!

Lai.

Manfred Spraul wrote:
> Based on the idea from Lai Jiangshan:
> 
> There are 23 instances where the rcu callback just does
> 
> 	kfree(containerof(rcu_member,struct whatever_struct,head));
> 
> The 23 instances exist because there are 23 'struct whatever_struct' with
> their individual rcu_member.
> 
> The attached patch creates a generic kfree_rcu function that removes the need
> for these 23 helpers: The offset from the container struct to the rcu member
> is calculated at compile time and stored in head->func instead of the
> function pointer.
> 
> 
> Effect on .text: (x86_64)
> +23 bytes in rcutree.c
> - 9 bytes in ipc/sem.c
> 
> Depending on the number of call_rcu instances in the .config, adding
> kfree_rcu should save between 100 and 200 bytes in .text.
> 
> What do you think?
> Is the interface ok? Incorrect use should generate compile time errors.
> Is it ok to use IS_ERR(p)?
> And: How many seperate patches should I create?
> I would propose 24 patches: one with the interface, 23 that
> replace the individual callbacks.
> 
> Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
> ---
>  include/linux/rcupdate.h |   33 +++++++++++++++++++++++++++++++++
>  ipc/sem.c                |   10 ++--------
>  kernel/rcuclassic.c      |    2 +-
>  kernel/rcupreempt.c      |    2 +-
>  kernel/rcutree.c         |    2 +-
>  5 files changed, 38 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 1168fbc..d120014 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -269,6 +269,29 @@ extern void call_rcu(struct rcu_head *head,
>  extern void call_rcu_bh(struct rcu_head *head,
>  			void (*func)(struct rcu_head *head));
>  
> +/**
> + * kfree_rcu - kfree after a grace period
> + * @ptr: pointer to kfree
> + * @rcu_member: name of the struct rcu_head member.
> + *
> + * Many rcu callbacks just call kfree() on the base structure. This helper
> + * function calls kfree internally. The rcu_head structure must be embedded
> + * in the to be freed structure.
> + */
> +
> +static inline void __kfree_rcu(void *prcu, unsigned long offset)
> +{
> +	void (*pfnc)(struct rcu_head *head) = (void (*)(struct rcu_head *head))offset;
> +
> +	BUILD_BUG_ON(!__builtin_constant_p(-offset));
> +	BUILD_BUG_ON(!IS_ERR(pfnc));
> +
> +	call_rcu(prcu, pfnc);
> +}
> +
> +#define kfree_rcu(pobj, entry) \
> +	__kfree_rcu(&((pobj)->entry), -offsetof(typeof(*(pobj)), entry))
> +
>  /* Exported common interfaces */
>  extern void synchronize_rcu(void);
>  extern void rcu_barrier(void);
> @@ -279,4 +302,14 @@ extern void rcu_barrier_sched(void);
>  extern void rcu_init(void);
>  extern int rcu_needs_cpu(int cpu);
>  
> +/* helper functions */
> +static inline void rcu_docallback(struct rcu_head *head)
> +{
> +	if (IS_ERR(head->func)) {
> +		kfree(((char *)head) + (unsigned long)head->func);
> +	} else {
> +		head->func(head);
> +	}
> +}
> +
>  #endif /* __LINUX_RCUPDATE_H */
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 0821224..4a68cf2 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -504,12 +504,6 @@ static int count_semzcnt (struct sem_array * sma, ushort semnum)
>  	return semzcnt;
>  }
>  
> -static void free_un(struct rcu_head *head)
> -{
> -	struct sem_undo *un = container_of(head, struct sem_undo, rcu);
> -	kfree(un);
> -}
> -
>  /* Free a semaphore set. freeary() is called with sem_ids.rw_mutex locked
>   * as a writer and the spinlock for this semaphore set hold. sem_ids.rw_mutex
>   * remains locked on exit.
> @@ -528,7 +522,7 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
>  		un->semid = -1;
>  		list_del_rcu(&un->list_proc);
>  		spin_unlock(&un->ulp->lock);
> -		call_rcu(&un->rcu, free_un);
> +		kfree_rcu(un, rcu);
>  	}
>  
>  	/* Wake up all pending processes and let them fail with EIDRM. */
> @@ -1347,7 +1341,7 @@ void exit_sem(struct task_struct *tsk)
>  		update_queue(sma);
>  		sem_unlock(sma);
>  
> -		call_rcu(&un->rcu, free_un);
> +		kfree_rcu(un, rcu);
>  	}
>  	kfree(ulp);
>  }
> diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
> index e503a00..b2f97d2 100644
> --- a/kernel/rcuclassic.c
> +++ b/kernel/rcuclassic.c
> @@ -336,7 +336,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>  	while (list) {
>  		next = list->next;
>  		prefetch(next);
> -		list->func(list);
> +		rcu_docallback(list);
>  		list = next;
>  		if (++count >= rdp->blimit)
>  			break;
> diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c
> index 0498265..8440dac 100644
> --- a/kernel/rcupreempt.c
> +++ b/kernel/rcupreempt.c
> @@ -1110,7 +1110,7 @@ static void rcu_process_callbacks(struct softirq_action *unused)
>  	spin_unlock_irqrestore(&rdp->lock, flags);
>  	while (list) {
>  		next = list->next;
> -		list->func(list);
> +		rcu_docallback(list);
>  		list = next;
>  		RCU_TRACE_ME(rcupreempt_trace_invoke);
>  	}
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index a342b03..29c88ce 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -901,7 +901,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>  	while (list) {
>  		next = list->next;
>  		prefetch(next);
> -		list->func(list);
> +		rcu_docallback(list);
>  		list = next;
>  		if (++count >= rdp->blimit)
>  			break;