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    SubjectRe: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2
    On Wed, Dec 07, 2011 at 11:19:16PM +0400, Cyrill Gorcunov wrote:
    ...
    > > >
    > > > This looks "obviously wrong".
    > > >
    > > > We can not trust ->children->next after rcu_read_unlock(). Another
    > > > rcu_read_lock() can't help.
    > > >
    > > > Once again, I can be easily wrong, need to read the patch first.
    > > >
    > >
    > > Wait, Oleg, I might be wrong as well, but it's now as
    > >
    > > children_seq_open
    > > get_proc_task (so ref to task increased)
    > >
    > > the children_seq_start/children_seq_stop works
    > > in iteration and every new iteration seq_list_next
    > > walks over the whole children list from the list
    > > head under rcu lock, so even if task is removed
    > > or added the link should exsist until rcu is unlocked
    > > and sync'ed no?
    > >
    >
    > On the other hands some if (task) tests are redundant
    > and might be dropped since we have a reference to a
    > task until seq-file is not released. I'll update it
    > and shrink a patch some more.
    >

    So while adding new task to a list is not a problem (the reader
    will simply not notice it) removing task from a list is a bit
    tricky, but as far as I see switch_task_namespaces (from exit_notify)
    uses synchronize_rcu as well as release_task calls for call_rcu
    for put_task_struct) so I think we will not get any wrong dereference
    in

    static int children_seq_show(struct seq_file *seq, void *v)
    {
    struct task_struct *task = container_of(v, struct task_struct, sibling);
    return seq_printf(seq, " %lu", (unsigned long)pid_vnr(task_pid(task)));
    }

    while we're in rcu reader section. I might be wrong of course, so
    please verify this claim.

    Cyrill


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