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Date: Tue, 6 Jun 2023 15:16:21 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 2/2 v2] 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.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
Is this better?
v1…v2:
- Extend the comment to note that preemption isn't disabled due to
the lock to make it obvious that the optimisation isn't just
harmful but also pointless.
kernel/signal.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
diff --git a/kernel/signal.c b/kernel/signal.c
index da017a5461163..dcb0b1fbcb3a8 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2328,11 +2328,20 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
* 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);
--
2.40.1
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