From d5541b6ef4eccee650abfe3095b9e7365773494c Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Wed, 22 Jun 2022 11:36:17 +0200 Subject: [PATCH 02/62] signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT. Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.1/older/patches-6.1.69-rt21.tar.xz Commit 53da1d9456fe7 ("fix ptrace slowness") is just band aid around the problem. The invocation of do_notify_parent_cldstop() wakes the parent and makes it runnable. The scheduler then wants to replace this still running task with the parent. With the read_lock() acquired this is not possible because preemption is disabled and so this is deferred until read_unlock(). This scheduling point is undesired and is avoided by disabling preemption around the unlock operation enabled again before the schedule() invocation without a preemption point. This is only undesired because the parent sleeps a cycle in wait_task_inactive() until the traced task leaves the run-queue in schedule(). It is not a correctness issue, it is just band aid to avoid the visbile delay which sums up over multiple invocations. The task can still be preempted if an interrupt occurs between preempt_enable_no_resched() and freezable_schedule() because on the IRQ-exit path of the interrupt scheduling _will_ happen. This is ignored since it does not happen very often. On PREEMPT_RT keeping preemption disabled during the invocation of cgroup_enter_frozen() becomes a problem because the function acquires css_set_lock which is a sleeping lock on PREEMPT_RT and must not be acquired with disabled preemption. Don't disable preemption on PREEMPT_RT. Remove the TODO regarding adding read_unlock_no_resched() as there is no need for it and will cause harm. Signed-off-by: Sebastian Andrzej Siewior Link: https://lkml.kernel.org/r/20220720154435.232749-2-bigeasy@linutronix.de --- kernel/signal.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/signal.c b/kernel/signal.c index 5d45f5da2b36..58e919c7c936 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -2302,13 +2302,13 @@ static int ptrace_stop(int exit_code, int why, unsigned long message, /* * Don't want to allow preemption here, because * sys_ptrace() needs this task to be inactive. - * - * XXX: implement read_unlock_no_resched(). */ - preempt_disable(); + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_disable(); read_unlock(&tasklist_lock); cgroup_enter_frozen(); - preempt_enable_no_resched(); + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_enable_no_resched(); schedule(); cgroup_leave_frozen(true); -- 2.43.0