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

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Commit 53da1d9456 ("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: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20230803100932.325870-2-bigeasy@linutronix.de
This commit is contained in:
Sebastian Andrzej Siewior 2023-08-03 12:09:31 +02:00 committed by Thomas Gleixner
parent 0bb80ecc33
commit a20d6f63db
1 changed files with 15 additions and 3 deletions
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18
kernel/signal.c
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@ -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);