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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 <oleg@redhat.com> Cc: Eric W. Biederman <ebiederm@xmission.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@ -3813,11 +3813,10 @@ static struct task_struct *first_tid(struct pid *pid, int tid, loff_t f_pos,
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/* If we haven't found our starting place yet start
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* with the leader and walk nr threads forward.
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*/
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pos = task = task->group_leader;
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do {
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for_each_thread(task, pos) {
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if (!nr--)
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goto found;
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} while_each_thread(task, pos);
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};
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fail:
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pos = NULL;
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goto out;
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@ -648,6 +648,10 @@ extern void flush_itimer_signals(void);
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extern bool current_is_single_threaded(void);
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/*
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* Without tasklist/siglock it is only rcu-safe if g can't exit/exec,
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* otherwise next_thread(t) will never reach g after list_del_rcu(g).
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*/
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#define while_each_thread(g, t) \
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while ((t = next_thread(t)) != g)
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