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From: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
To: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        John Stultz <john.stultz@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>, Oleg Nesterov <oleg@redhat.com>,
        Nicolas Pitre <nicolas.pitre@linaro.org>,
        Michal Hocko <mhocko@suse.com>,
        Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>,
        Mateusz Guzik <mguzik@redhat.com>, linux-kernel@vger.kernel.org,
        Pavel Emelyanov <xemul@virtuozzo.com>,
        Konstantin Khorenko <khorenko@virtuozzo.com>,
        Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
        Lennart Poettering <lennart@poettering.net>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Kay Sievers <kay.sievers@vrfy.org>
Subject: [PATCH 1/2] introduce the walk_process_tree() helper
Date: Fri, 27 Jan 2017 13:05:42 +0300
Message-Id: <20170127100543.18390-2-ptikhomirov@virtuozzo.com>
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From: Oleg Nesterov <oleg@redhat.com>

Add the new helper to walk the process tree, the next patch adds a user.
Note that it visits the group leaders only, proc_visitor can do
for_each_thread itself or we can trivially extend walk_process_tree() to
do this.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
---
 include/linux/sched.h |  3 +++
 kernel/fork.c         | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ad3ec9e..7f8ab91 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -3067,6 +3067,9 @@ extern bool current_is_single_threaded(void);
 #define for_each_process_thread(p, t)	\
 	for_each_process(p) for_each_thread(p, t)
 
+typedef int (*proc_visitor)(struct task_struct *p, void *data);
+void walk_process_tree(struct task_struct *top, proc_visitor, void *);
+
 static inline int get_nr_threads(struct task_struct *tsk)
 {
 	return tsk->signal->nr_threads;
diff --git a/kernel/fork.c b/kernel/fork.c
index 11c5c8a..135b7a4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2053,6 +2053,38 @@ SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
 }
 #endif
 
+void walk_process_tree(struct task_struct *top, proc_visitor visitor, void *data)
+{
+	struct task_struct *leader, *parent, *child;
+	int res;
+
+	read_lock(&tasklist_lock);
+	leader = top = top->group_leader;
+down:
+	for_each_thread(leader, parent) {
+		list_for_each_entry(child, &parent->children, sibling) {
+			res = visitor(child, data);
+			if (res) {
+				if (res < 0)
+					goto out;
+				leader = child;
+				goto down;
+			}
+up:
+			;
+		}
+	}
+
+	if (leader != top) {
+		child = leader;
+		parent = child->real_parent;
+		leader = parent->group_leader;
+		goto up;
+	}
+out:
+	read_unlock(&tasklist_lock);
+}
+
 #ifndef ARCH_MIN_MMSTRUCT_ALIGN
 #define ARCH_MIN_MMSTRUCT_ALIGN 0
 #endif
-- 
2.9.3