Commit a20d6f63 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Thomas Gleixner
Browse files

signal: Add a proper comment about preempt_disable() in ptrace_stop() 



Commit 53da1d94 ("fix ptrace slowness") added a preempt-disable section
between read_unlock() and the following schedule() invocation without
explaining why it is needed.

Replace the existing contentless comment with a proper explanation to
clarify that it is not needed for correctness but for performance reasons.

Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarOleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20230803100932.325870-2-bigeasy@linutronix.de
parent 0bb80ecc

kernel/signal.c

+15 −3
Original line number Diff line number Diff line
@@ -2329,10 +2329,22 @@ static int ptrace_stop(int exit_code, int why, unsigned long message, 
		do_notify_parent_cldstop(current, false, why);



	/*

	 * Don't want to allow preemption here, because

	 * sys_ptrace() needs this task to be inactive.

	 * The previous do_notify_parent_cldstop() invocation woke ptracer.

	 * One a PREEMPTION kernel this can result in preemption requirement

	 * which will be fulfilled after read_unlock() and the ptracer will be

	 * put on the CPU.

	 * The ptracer is in wait_task_inactive(, __TASK_TRACED) waiting for

	 * this task wait in schedule(). If this task gets preempted then it

	 * remains enqueued on the runqueue. The ptracer will observe this and

	 * then sleep for a delay of one HZ tick. In the meantime this task

	 * gets scheduled, enters schedule() and will wait for the ptracer.

	 *

	 * XXX: implement read_unlock_no_resched().

	 * This preemption point is not bad from a correctness point of

	 * view but extends the runtime by one HZ tick time due to the

	 * ptracer's sleep.  The preempt-disable section ensures that there

	 * will be no preemption between unlock and schedule() and so

	 * improving the performance since the ptracer will observe that

	 * the tracee is scheduled out once it gets on the CPU.

	 */

	preempt_disable();

	read_unlock(&tasklist_lock);