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    SubjectRe: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2
    On Thu, Dec 08, 2011 at 05:35:35PM +0100, Oleg Nesterov wrote:
    ...
    >
    > However, ->children list is not rcu-safe, this means that even
    > list_for_each() itself is not safe. Either you need tasklist or
    > we can probably make it rcu-safe...
    >

    Andrew, Oleg, does the below one look more less fine? Note the
    tasklist_lock is back and it worries me a bit since I imagine
    one could be endlessly reading some /proc/<pid>/children file
    increasing contention over this lock on the whole system
    (regardless the fact that it's take for read only).

    Cyrill
    ---
    From: Cyrill Gorcunov <gorcunov@openvz.org>
    Subject: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v4

    There is no easy way to make a reverse parent->children chain
    from arbitrary <pid> (while parent pid is provided in "PPid"
    field of /proc/<pid>/status).

    So instead of walking over all pids in the system to figure out which
    children a task have -- we add explicit /proc/<pid>/children entry,
    because kernel already has this kind of information but it is not
    yet exported. This is a first level children, not the whole process
    tree, neither the process threads are identified with this interface.

    v2:
    - Kame suggested to use a separated /proc/<pid>/children entry
    instead of poking /proc/<pid>/status
    - Andew suggested to use rcu facility instead of locking
    tasklist_lock
    - Tejun pointed that non-seekable seq file might not be
    enough for tasks with large number of children

    v3:
    - To be on a safe side use %lu format for pid_t printing

    v4:
    - New line get printed when sequence ends not at seq->stop,
    a nit pointed by Tejun
    - Documentation update
    - tasklist_lock is back, Oleg pointed that ->children list
    is actually not rcu-safe

    Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Pavel Emelyanov <xemul@parallels.com>
    Cc: Serge Hallyn <serge.hallyn@canonical.com>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Cc: Pavel Emelyanov <xemul@parallels.com>
    ---
    Documentation/filesystems/proc.txt | 11 +++
    fs/proc/array.c | 127 +++++++++++++++++++++++++++++++++++++
    fs/proc/base.c | 1
    fs/proc/internal.h | 6 +
    4 files changed, 145 insertions(+)

    Index: linux-2.6.git/Documentation/filesystems/proc.txt
    ===================================================================
    --- linux-2.6.git.orig/Documentation/filesystems/proc.txt
    +++ linux-2.6.git/Documentation/filesystems/proc.txt
    @@ -40,6 +40,7 @@ Table of Contents
    3.4 /proc/<pid>/coredump_filter - Core dump filtering settings
    3.5 /proc/<pid>/mountinfo - Information about mounts
    3.6 /proc/<pid>/comm & /proc/<pid>/task/<tid>/comm
    + 3.7 /proc/<pid>/children - Information about task children


    ------------------------------------------------------------------------------
    @@ -1545,3 +1546,13 @@ a task to set its own or one of its thre
    is limited in size compared to the cmdline value, so writing anything longer
    then the kernel's TASK_COMM_LEN (currently 16 chars) will result in a truncated
    comm value.
    +
    +3.7 /proc/<pid>/children - Information about task children
    +--------------------------------------------------------------
    +This file provides a fast way to retrieve first level children pids
    +of a task pointed by <pid>. The format is a stream of pids separated
    +by space with a new line at the end. If a task has no children at
    +all -- only a new line returned. Note the "first level" here -- if
    +a task child has own children they will not be printed there, one
    +need to read /proc/<children-pid>/children to obtain such pids.
    +The same applies to threads -- they are not counted here.
    Index: linux-2.6.git/fs/proc/array.c
    ===================================================================
    --- linux-2.6.git.orig/fs/proc/array.c
    +++ linux-2.6.git/fs/proc/array.c
    @@ -547,3 +547,130 @@ int proc_pid_statm(struct seq_file *m, s

    return 0;
    }
    +
    +static struct list_head *
    +children_get_at(struct proc_pid_children_iter *iter, loff_t pos)
    +{
    + struct task_struct *t = iter->leader;
    + do {
    + struct list_head *p;
    + list_for_each(p, &t->children) {
    + if (pos-- == 0) {
    + iter->last = t;
    + return p;
    + }
    + }
    + } while_each_thread(iter->leader, t);
    +
    + /* To make sure it's never in unknown state */
    + iter->last = NULL;
    +
    + return NULL;
    +}
    +
    +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)));
    +}
    +
    +static void *children_seq_start(struct seq_file *seq, loff_t *pos)
    +{
    + struct proc_pid_children_iter *iter = seq->private;
    +
    + read_lock(&tasklist_lock);
    + return children_get_at(iter, *pos);
    +}
    +
    +static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
    +{
    + struct proc_pid_children_iter *iter = seq->private;
    + struct list_head *next = NULL;
    +
    + if (iter->last) {
    + if (list_is_last(v, &iter->last->children)) {
    + while_each_thread(iter->leader, iter->last) {
    + if (!list_empty(&iter->last->children)) {
    + next = iter->last->children.next;
    + goto found;
    + }
    + }
    + iter->last = NULL;
    + } else
    + next = ((struct list_head *)v)->next;
    + }
    +
    +found:
    + ++*pos;
    + if (!next)
    + seq_printf(seq, "\n");
    + return next;
    +}
    +
    +static void children_seq_stop(struct seq_file *seq, void *v)
    +{
    + struct proc_pid_children_iter *iter = seq->private;
    +
    + iter->last = NULL;
    + read_unlock(&tasklist_lock);
    +}
    +
    +static const struct seq_operations children_seq_ops = {
    + .start = children_seq_start,
    + .next = children_seq_next,
    + .stop = children_seq_stop,
    + .show = children_seq_show,
    +};
    +
    +static int children_seq_open(struct inode *inode, struct file *file)
    +{
    + struct proc_pid_children_iter *iter = NULL;
    + struct task_struct *task = NULL;
    + int ret;
    +
    + ret = -ENOMEM;
    + iter = kmalloc(sizeof(*iter), GFP_KERNEL);
    + if (!iter)
    + goto err;
    +
    + ret = -ENOENT;
    + task = get_proc_task(inode);
    + if (!task)
    + goto err;
    +
    + ret = seq_open(file, &children_seq_ops);
    + if (!ret) {
    + struct seq_file *m = file->private_data;
    + m->private = iter;
    + iter->leader = task;
    + iter->last = task;
    + }
    +
    +err:
    + if (ret) {
    + if (task)
    + put_task_struct(task);
    + kfree(iter);
    + }
    +
    + return ret;
    +}
    +
    +int children_seq_release(struct inode *inode, struct file *file)
    +{
    + struct seq_file *m = file->private_data;
    + struct proc_pid_children_iter *iter = m->private;
    +
    + put_task_struct(iter->leader);
    + kfree(iter);
    + seq_release(inode, file);
    +
    + return 0;
    +}
    +
    +const struct file_operations proc_pid_children_operations = {
    + .open = children_seq_open,
    + .read = seq_read,
    + .llseek = seq_lseek,
    + .release = children_seq_release,
    +};
    Index: linux-2.6.git/fs/proc/base.c
    ===================================================================
    --- linux-2.6.git.orig/fs/proc/base.c
    +++ linux-2.6.git/fs/proc/base.c
    @@ -3204,6 +3204,7 @@ static const struct pid_entry tgid_base_
    INF("cmdline", S_IRUGO, proc_pid_cmdline),
    ONE("stat", S_IRUGO, proc_tgid_stat),
    ONE("statm", S_IRUGO, proc_pid_statm),
    + REG("children", S_IRUGO, proc_pid_children_operations),
    REG("maps", S_IRUGO, proc_maps_operations),
    #ifdef CONFIG_NUMA
    REG("numa_maps", S_IRUGO, proc_numa_maps_operations),
    Index: linux-2.6.git/fs/proc/internal.h
    ===================================================================
    --- linux-2.6.git.orig/fs/proc/internal.h
    +++ linux-2.6.git/fs/proc/internal.h
    @@ -53,6 +53,12 @@ extern int proc_pid_statm(struct seq_fil
    struct pid *pid, struct task_struct *task);
    extern loff_t mem_lseek(struct file *file, loff_t offset, int orig);

    +struct proc_pid_children_iter {
    + struct task_struct *leader;
    + struct task_struct *last;
    +};
    +
    +extern const struct file_operations proc_pid_children_operations;
    extern const struct file_operations proc_maps_operations;
    extern const struct file_operations proc_numa_maps_operations;
    extern const struct file_operations proc_smaps_operations;

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