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document while_each_thread(), change first_tid() to use for_each_thread()

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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 <oleg@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2023-08-23 19:08:06 +02:00 committed by Andrew Morton
parent ed1af26cd2
commit dce8f8ed1d
2 changed files with 6 additions and 3 deletions
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5
fs/proc/base.c
View File

@ -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;

4
include/linux/sched/signal.h
View File

@ -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)