Hello,
On Mon, Jul 11, 2016 at 01:32:06PM -0400, Waiman Long wrote:
...
> A new header file include/linux/dlock-list.h will be added with the
Heh, I think perpcu_list was the better name but suppose I'm too late.
> associated dlock_list_head and dlock_list_node structures. The following
> functions are provided to manage the per-cpu list:
>
> 1. int init_dlock_list_head(struct dlock_list_head **pdlock_head)
> 2. void dlock_list_add(struct dlock_list_node *node,
> struct dlock_list_head *head)
> 3. void dlock_list_del(struct dlock_list *node)
>
> Iteration of all the list entries within a group of per-cpu
> lists is done by calling either the dlock_list_iterate() or
> dlock_list_iterate_safe() functions in a while loop. They correspond
> to the list_for_each_entry() and list_for_each_entry_safe() macros
> respectively. The iteration states are keep in a dlock_list_state
> structure that is passed to the iteration functions.
Why do we need two variants of this? We need a state variable to walk
the list anyway. Why not make dlock_list_iterate() safe against
removal and get rid of the second variant? Also, dlock_list_next()
probably is a better name.
> +/*
> + * include/linux/dlock-list.h
> + *
> + * A distributed (per-cpu) set of lists each of which is protected by its
> + * own spinlock, but acts like a single consolidated list to the callers.
> + *
> + * The dlock_list_head structure contains the spinlock, the other
> + * dlock_list_node structures only contains a pointer to the spinlock in
> + * dlock_list_head.
> + */
> +struct dlock_list_head {
> + struct list_head list;
> + spinlock_t lock;
> +};
> +
> +#define DLOCK_LIST_HEAD_INIT(name) \
> + { \
> + .list.prev = &name.list, \
> + .list.next = &name.list, \
> + .list.lock = __SPIN_LOCK_UNLOCKED(name), \
> + }
This is confusing. It looks like dlock_list_head and
DLOCK_LIST_HEAD_INIT() can be used to define and initialize static
dlock_lists but that isn't true. It's weird to require the user to
deal with percpu declaration of the data type. Shouldn't it be more
like the following?
struct dlock_list_head_cpu {
struct list_head list;
spinlock_t lock;
};
struct dlock_list_head {
struct dlock_list_head_percpu *head_cpu;
};
> +/*
> + * Per-cpu list iteration state
> + */
> +struct dlock_list_state {
> + int cpu;
> + spinlock_t *lock;
> + struct list_head *head; /* List head of current per-cpu list */
> + struct dlock_list_node *curr;
> + struct dlock_list_node *next;
> +};
Maybe dlock_list_iter[ator] is a better name?
> +#define DLOCK_LIST_STATE_INIT() \
> + { \
> + .cpu = -1, \
> + .lock = NULL, \
> + .head = NULL, \
> + .curr = NULL, \
> + .next = NULL, \
> + }
The NULL inits are unnecessary and prone to being left behind.
> +#define DEFINE_DLOCK_LIST_STATE(s) \
> + struct dlock_list_state s = DLOCK_LIST_STATE_INIT()
> +
> +static inline void init_dlock_list_state(struct dlock_list_state *state)
> +{
> + state->cpu = -1;
> + state->lock = NULL;
> + state->head = NULL;
> + state->curr = NULL;
> + state->next = NULL;
> +}
Why not "state = (struct dlock_list_state)DLOCK_LIST_STATE_INIT;"?
> +#ifdef CONFIG_DEBUG_SPINLOCK
> +#define DLOCK_LIST_WARN_ON(x) WARN_ON(x)
> +#else
> +#define DLOCK_LIST_WARN_ON(x)
> +#endif
I'd just use WARN_ON_ONCE() without the CONFIG guard.
> +/*
> + * Next per-cpu list entry
> + */
> +#define dlock_list_next_entry(pos, member) list_next_entry(pos, member.list)
Why does this need to be exposed?
> +/*
> + * Per-cpu node data structure
> + */
> +struct dlock_list_node {
> + struct list_head list;
> + spinlock_t *lockptr;
> +};
> +
> +#define DLOCK_LIST_NODE_INIT(name) \
> + { \
> + .list.prev = &name.list, \
> + .list.next = &name.list, \
> + .list.lockptr = NULL \
> + }
Ditto with NULL init.
> +static inline void init_dlock_list_node(struct dlock_list_node *node)
> +{
> + INIT_LIST_HEAD(&node->list);
> + node->lockptr = NULL;
> +}
Ditto with init.
> +static inline void free_dlock_list_head(struct dlock_list_head **pdlock_head)
> +{
> + free_percpu(*pdlock_head);
> + *pdlock_head = NULL;
> +}
Why does this need to be inlined?
> +/*
> + * Check if all the per-cpu lists are empty
> + */
Please use proper function comments.
> +static inline bool dlock_list_empty(struct dlock_list_head *dlock_head)
> +{
> + int cpu;
> +
> + for_each_possible_cpu(cpu)
> + if (!list_empty(&per_cpu_ptr(dlock_head, cpu)->list))
> + return false;
> + return true;
> +}
> +
> +/*
> + * Helper function to find the first entry of the next per-cpu list
> + * It works somewhat like for_each_possible_cpu(cpu).
> + *
> + * Return: true if the entry is found, false if all the lists exhausted
Ditto about the comment.
> + */
> +static __always_inline bool
> +__dlock_list_next_cpu(struct dlock_list_head *head,
> + struct dlock_list_state *state)
> +{
...
> +static inline bool dlock_list_iterate_safe(struct dlock_list_head *head,
> + struct dlock_list_state *state)
> +{
Inlining these doesn't make senes to me.
> diff --git a/lib/Makefile b/lib/Makefile
> index 499fb35..92e8c38 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> +/*
> + * The dlock list lock needs its own class to avoid warning and stack
> + * trace when lockdep is enabled.
> + */
Can you please elaborate on this?
> +static struct lock_class_key dlock_list_key;
> +
> +/*
> + * Initialize the per-cpu list head
> + */
> +int init_dlock_list_head(struct dlock_list_head **pdlock_head)
> +{
> + struct dlock_list_head *dlock_head;
> + int cpu;
> +
> + dlock_head = alloc_percpu(struct dlock_list_head);
> + if (!dlock_head)
> + return -ENOMEM;
> +
> + for_each_possible_cpu(cpu) {
> + struct dlock_list_head *head = per_cpu_ptr(dlock_head, cpu);
> +
> + INIT_LIST_HEAD(&head->list);
> + head->lock = __SPIN_LOCK_UNLOCKED(&head->lock);
> + lockdep_set_class(&head->lock, &dlock_list_key);
> + }
> +
> + *pdlock_head = dlock_head;
> + return 0;
> +}
Why is this called init? Why not do the following?
struct dlock_list_head *alloc_dlock_list_head(void);
Also, the pointer type needs to include __percpu annotation.
> +/*
> + * List selection is based on the CPU being used when the dlock_list_add()
> + * function is called. However, deletion may be done by a different CPU.
> + * So we still need to use a lock to protect the content of the list.
> + */
> +void dlock_list_add(struct dlock_list_node *node, struct dlock_list_head *head)
> +{
> + struct dlock_list_head *myhead;
> +
> + /*
> + * Disable preemption to make sure that CPU won't gets changed.
> + */
> + myhead = get_cpu_ptr(head);
> + spin_lock(&myhead->lock);
> + node->lockptr = &myhead->lock;
> + list_add(&node->list, &myhead->list);
> + spin_unlock(&myhead->lock);
> + put_cpu_ptr(head);
> +}
I wonder whether it'd be better to use irqsafe operations. lists tend
to be often used from irq contexts.
> +/*
> + * Delete a node from a dlock list
> + *
> + * We need to check the lock pointer again after taking the lock to guard
> + * against concurrent delete of the same node. If the lock pointer changes
> + * (becomes NULL or to a different one), we assume that the deletion was done
> + * elsewhere.
> + */
> +void dlock_list_del(struct dlock_list_node *node)
> +{
> + spinlock_t *lock = READ_ONCE(node->lockptr);
> +
> + if (unlikely(!lock)) {
> + WARN(1, "dlock_list_del: node 0x%lx has no associated lock\n",
> + (unsigned long)node);
> + return;
> + }
> +
> + spin_lock(lock);
> + if (likely(lock == node->lockptr)) {
> + list_del_init(&node->list);
> + node->lockptr = NULL;
> + } else {
> + /*
> + * This path should never be executed.
> + */
What if it races against someone else removing and adding back?
Shouldn't it retry on those cases?
Thanks.
--
tejun
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