Re: [PATCH v6 1/4] lib/percpu-list: Per-cpu list with associated per-cpu locks

From: Waiman Long
Date: Mon Mar 21 2016 - 22:31:42 EST

On 03/21/2016 05:49 AM, Jan Kara wrote:
On Fri 18-03-16 15:44:01, Waiman Long wrote:
+static __always_inline bool
+__pcpu_list_next_cpu(struct pcpu_list_head *head, struct pcpu_list_state *state)
+ if (state->lock)
+ spin_unlock(state->lock);
+ /*
+ * for_each_possible_cpu(cpu)
+ */
+ state->cpu = cpumask_next(state->cpu, cpu_possible_mask);
+ if (state->cpu>= nr_cpu_ids)
+ return false; /* All the per-cpu lists iterated */
+ state->head =&per_cpu_ptr(head, state->cpu)->list;
+ if (list_empty(state->head))
+ goto next_cpu;
+ state->lock =&per_cpu_ptr(head, state->cpu)->lock;
+ spin_lock(state->lock);
+ state->curr = list_entry(state->head->next,
+ struct pcpu_list_node, list);
+ return true;
Waiman, I repeat it for the third time as you keep ignoring it: This is
*racy*. The list for state->cpu can be empty by the time you acquire
state->lock and thus state->curr will point somewhere around the head of
the list but definitely not to a list member where it should.


I am sorry for missing your previous comment. Yes, it is possible that the list is empty after the lock. So I should have checked for that before returning. Thanks for reminding me that. I will fix that later on.