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| author | Sebastian Andrzej Siewior <bigeasy@linutronix.de> | 2023-08-03 13:09:31 +0300 |
|---|---|---|
| committer | Thomas Gleixner <tglx@linutronix.de> | 2023-09-19 23:08:29 +0300 |
| commit | a20d6f63dbfc176697886d7709312ad0a795648e (patch) | |
| tree | 7fefa92d3aa507eb7e9f89cba946e10dbf1b056b /kernel/signal.c | |
| parent | 0bb80ecc33a8fb5a682236443c1e740d5c917d1d (diff) | |
| download | linux-a20d6f63dbfc176697886d7709312ad0a795648e.tar.xz | |
signal: Add a proper comment about preempt_disable() in ptrace_stop()
Commit 53da1d9456fe7 ("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
Diffstat (limited to 'kernel/signal.c')
| -rw-r--r-- | kernel/signal.c | 18 |
1 files changed, 15 insertions, 3 deletions
diff --git a/kernel/signal.c b/kernel/signal.c index 09019017d669..3035bebd7075 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -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); |