[tip: core/core] signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT

[tip-bot2 for Sebastian Andrzej Siewior]

The following commit has been merged into the core/core branch of tip:

Commit-ID:     1aabbc532413ced293952f8e149ad0a607d6e470
Gitweb:        https://git.kernel.org/tip/1aabbc532413ced293952f8e149ad0a607d6e470
Author:        Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
AuthorDate:    Thu, 03 Aug 2023 12:09:32 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 19 Sep 2023 22:08:29 +02:00

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@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230803100932.325870-3-bigeasy@xxxxxxxxxxxxx

---
 kernel/signal.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 3035beb..f2a5578 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2345,11 +2345,22 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
 	 * 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.
+	 *
+	 * 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.
+	 *
+	 * In fact disabling preemption is harmful on PREEMPT_RT because
+	 * the spinlock_t in cgroup_enter_frozen() must not be acquired
+	 * with preemption disabled due to the 'sleeping' spinlock
+	 * substitution of RT.
 	 */
-	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);