66 lines
2.5 KiB
Diff
66 lines
2.5 KiB
Diff
From 2defd6085e9803ba06f6b56f6f901309462761a6 Mon Sep 17 00:00:00 2001
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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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Date: Wed, 22 Jun 2022 11:36:17 +0200
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Subject: [PATCH 02/62] signal: Don't disable preemption in ptrace_stop() on
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PREEMPT_RT.
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Commit
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53da1d9456fe7 ("fix ptrace slowness")
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is just band aid around the problem.
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The invocation of do_notify_parent_cldstop() wakes the parent and makes
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it runnable. The scheduler then wants to replace this still running task
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with the parent. With the read_lock() acquired this is not possible
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because preemption is disabled and so this is deferred until read_unlock().
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This scheduling point is undesired and is avoided by disabling preemption
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around the unlock operation enabled again before the schedule() invocation
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without a preemption point.
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This is only undesired because the parent sleeps a cycle in
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wait_task_inactive() until the traced task leaves the run-queue in
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schedule(). It is not a correctness issue, it is just band aid to avoid the
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visbile delay which sums up over multiple invocations.
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The task can still be preempted if an interrupt occurs between
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preempt_enable_no_resched() and freezable_schedule() because on the IRQ-exit
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path of the interrupt scheduling _will_ happen. This is ignored since it does
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not happen very often.
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On PREEMPT_RT keeping preemption disabled during the invocation of
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cgroup_enter_frozen() becomes a problem because the function acquires
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css_set_lock which is a sleeping lock on PREEMPT_RT and must not be
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acquired with disabled preemption.
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Don't disable preemption on PREEMPT_RT. Remove the TODO regarding adding
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read_unlock_no_resched() as there is no need for it and will cause harm.
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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Link: https://lkml.kernel.org/r/20220720154435.232749-2-bigeasy@linutronix.de
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---
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kernel/signal.c | 8 ++++----
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1 file changed, 4 insertions(+), 4 deletions(-)
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diff --git a/kernel/signal.c b/kernel/signal.c
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index 5d45f5da2b36..58e919c7c936 100644
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--- a/kernel/signal.c
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+++ b/kernel/signal.c
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@@ -2302,13 +2302,13 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
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/*
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* Don't want to allow preemption here, because
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* sys_ptrace() needs this task to be inactive.
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- *
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- * XXX: implement read_unlock_no_resched().
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*/
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- preempt_disable();
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+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
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+ preempt_disable();
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read_unlock(&tasklist_lock);
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cgroup_enter_frozen();
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- preempt_enable_no_resched();
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+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
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+ preempt_enable_no_resched();
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schedule();
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cgroup_leave_frozen(true);
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--
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2.43.0
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