Commit 8feebc67 authored Apr 27, 2020 by Eric W. Biederman

posix-cpu-timer: Tidy up group_leader logic in lookup_task

Replace has_group_leader_pid with thread_group_leader. Years ago Oleg
suggested changing thread_group_leader to has_group_leader_pid to handle
races. Looking at the code then and now I don't see how it ever helped.
Especially as then the code really did need to be the
thread_group_leader.

Today it doesn't make a difference if thread_group_leader races with
de_thread as the task returned from lookup_task in the non-thread case is
just used to find values in task->signal.

Since the races with de_thread have never been handled revert
has_group_header_pid to thread_group_leader for clarity.

Update the comment in lookup_task to remove implementation details that
are no longer true and to mention task->signal instead of task->sighand,
as the relevant cpu timer details are all in task->signal.

Ref: 55e8c8eb ("posix-cpu-timers: Store a reference to a pid not a task")
Ref: c0deae8c ("posix-cpu-timers: Rcu_read_lock/unlock protect find_task_by_vpid call")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

parent 50712280

kernel/time/posix-cpu-timers.c

+2 −6

Original line number	Diff line number	Diff line
		@@ -69,12 +69,8 @@ static struct task_struct *lookup_task(const pid_t pid, bool thread,
		if (gettime) {
		/*
		* For clock_gettime(PROCESS) the task does not need to be
		* the actual group leader. tsk->sighand gives
		* the actual group leader. task->signal gives
		* access to the group's clock.
		*
		* Timers need the group leader because they take a
		* reference on it and store the task pointer until the
		* timer is destroyed.
		*/
		return (p == current \|\| thread_group_leader(p)) ? p : NULL;
		}
		@@ -82,7 +78,7 @@ static struct task_struct *lookup_task(const pid_t pid, bool thread,
		/*
		* For processes require that p is group leader.
		*/
		return has_group_leader_pid(p) ? p : NULL;
		return thread_group_leader(p) ? p : NULL;
		}

		static struct task_struct *__get_task_for_clock(const clockid_t clock,

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