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Subject[PATCH] Fixes for RCU handling of task_struct
Hello!

My earlier code that applies RCU to the task list (in PREEMPT_RT)
was missing some rcu_dereference() and rcu_assign_pointer() calls.
This patch fixes these problems.

Signed-off-by: <paulmck@us.ibm.com>

---
include/linux/list.h | 20 ++++++++++++++++++++
kernel/pid.c | 20 ++++++++++----------
2 files changed, 30 insertions(+), 10 deletions(-)
diff -urpNa -X dontdiff linux-2.6.14-rc5-rt2/include/linux/list.h linux-2.6.14-rc5-rt2-RCUusefix/include/linux/list.h
--- linux-2.6.14-rc5-rt2/include/linux/list.h 2005-10-22 14:41:46.000000000 -0700
+++ linux-2.6.14-rc5-rt2-RCUusefix/include/linux/list.h 2005-10-27 11:02:53.000000000 -0700
@@ -208,6 +208,7 @@ static inline void list_replace_rcu(stru
smp_wmb();
new->next->prev = new;
new->prev->next = new;
+ old->prev = LIST_POISON2;
}

/**
@@ -578,6 +579,25 @@ static inline void hlist_del_init(struct
}
}

+/*
+ * hlist_replace_rcu - replace old entry by new one
+ * @old : the element to be replaced
+ * @new : the new element to insert
+ *
+ * The old entry will be replaced with the new entry atomically.
+ */
+static inline void hlist_replace_rcu(struct hlist_node *old, struct hlist_node *new){
+ struct hlist_node *next = old->next;
+
+ new->next = next;
+ new->pprev = old->pprev;
+ smp_wmb();
+ if (next)
+ new->next->pprev = &new->next;
+ *new->pprev = new;
+ old->pprev = LIST_POISON2;
+}
+
static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
{
struct hlist_node *first = h->first;
diff -urpNa -X dontdiff linux-2.6.14-rc5-rt2/kernel/pid.c linux-2.6.14-rc5-rt2-RCUusefix/kernel/pid.c
--- linux-2.6.14-rc5-rt2/kernel/pid.c 2005-10-22 14:45:56.000000000 -0700
+++ linux-2.6.14-rc5-rt2-RCUusefix/kernel/pid.c 2005-10-27 11:12:15.000000000 -0700
@@ -136,7 +136,7 @@ struct pid * fastcall find_pid(enum pid_
struct hlist_node *elem;
struct pid *pid;

- hlist_for_each_entry(pid, elem,
+ hlist_for_each_entry_rcu(pid, elem,
&pid_hash[type][pid_hashfn(nr)], pid_chain) {
if (pid->nr == nr)
return pid;
@@ -151,12 +151,12 @@ int fastcall attach_pid(task_t *task, en
task_pid = &task->pids[type];
pid = find_pid(type, nr);
if (pid == NULL) {
- hlist_add_head(&task_pid->pid_chain,
- &pid_hash[type][pid_hashfn(nr)]);
INIT_LIST_HEAD(&task_pid->pid_list);
+ hlist_add_head_rcu(&task_pid->pid_chain,
+ &pid_hash[type][pid_hashfn(nr)]);
} else {
INIT_HLIST_NODE(&task_pid->pid_chain);
- list_add_tail(&task_pid->pid_list, &pid->pid_list);
+ list_add_tail_rcu(&task_pid->pid_list, &pid->pid_list);
}
task_pid->nr = nr;

@@ -170,20 +170,20 @@ static fastcall int __detach_pid(task_t

pid = &task->pids[type];
if (!hlist_unhashed(&pid->pid_chain)) {
- hlist_del(&pid->pid_chain);

- if (list_empty(&pid->pid_list))
+ if (list_empty(&pid->pid_list)) {
nr = pid->nr;
- else {
+ hlist_del_rcu(&pid->pid_chain);
+ } else {
pid_next = list_entry(pid->pid_list.next,
struct pid, pid_list);
/* insert next pid from pid_list to hash */
- hlist_add_head(&pid_next->pid_chain,
- &pid_hash[type][pid_hashfn(pid_next->nr)]);
+ hlist_replace_rcu(&pid->pid_chain,
+ &pid_next->pid_chain);
}
}

- list_del(&pid->pid_list);
+ list_del_rcu(&pid->pid_list);
pid->nr = 0;

return nr;
-
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