From dce8f8ed1de1d9d6d27c5ccd202ce4ec163b100c Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Wed, 23 Aug 2023 19:08:06 +0200 Subject: [PATCH] document while_each_thread(), change first_tid() to use for_each_thread() Add the comment to explain that while_each_thread(g,t) is not rcu-safe unless g is stable (e.g. current). Even if g is a group leader and thus can't exit before t, t or another sub-thread can exec and remove g from the thread_group list. The only lockless user of while_each_thread() is first_tid() and it is fine in that it can't loop forever, yet for_each_thread() looks better and I am going to change while_each_thread/next_thread. Link: https://lkml.kernel.org/r/20230823170806.GA11724@redhat.com Signed-off-by: Oleg Nesterov Cc: Eric W. Biederman Signed-off-by: Andrew Morton --- fs/proc/base.c | 5 ++--- include/linux/sched/signal.h | 4 ++++ 2 files changed, 6 insertions(+), 3 deletions(-) diff --git a/fs/proc/base.c b/fs/proc/base.c index 05452c3b9872..483a3edebdd1 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -3813,11 +3813,10 @@ static struct task_struct *first_tid(struct pid *pid, int tid, loff_t f_pos, /* If we haven't found our starting place yet start * with the leader and walk nr threads forward. */ - pos = task = task->group_leader; - do { + for_each_thread(task, pos) { if (!nr--) goto found; - } while_each_thread(task, pos); + }; fail: pos = NULL; goto out; diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index 0deebe2ab07d..0014d3adaf84 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -648,6 +648,10 @@ extern void flush_itimer_signals(void); extern bool current_is_single_threaded(void); +/* + * Without tasklist/siglock it is only rcu-safe if g can't exit/exec, + * otherwise next_thread(t) will never reach g after list_del_rcu(g). + */ #define while_each_thread(g, t) \ while ((t = next_thread(t)) != g)