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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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