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Message-Id: <20230803100932.325870-3-bigeasy@linutronix.de>
Date: Thu, 3 Aug 2023 12:09:32 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
Oleg Nesterov <oleg@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 2/2] signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT.
On PREEMPT_RT keeping preemption disabled during the invocation of
cgroup_enter_frozen() is 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.
The preempt-disabled section is only for performance optimisation
reasons and can be avoided.
Extend the comment and don't disable preemption before scheduling on
PREEMPT_RT.
Acked-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
kernel/signal.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2333,11 +2333,20 @@ static int ptrace_stop(int exit_code, in
* The preempt-disable section ensures that there will be no preemption
* between unlock and schedule() and so improving the performance since
* the ptracer has no reason to sleep.
+ *
+ * On PREEMPT_RT locking tasklist_lock does not disable preemption.
+ * Therefore the task can be preempted (after
+ * do_notify_parent_cldstop()) before unlocking tasklist_lock so there
+ * is no benefit in doing this. The optimisation is harmful on
+ * PEEMPT_RT because the spinlock_t (in cgroup_enter_frozen()) must not
+ * be acquired with disabled preemption.
*/
- 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);
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