Commit 1aabbc53 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Thomas Gleixner
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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: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarOleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20230803100932.325870-3-bigeasy@linutronix.de
parent a20d6f63

kernel/signal.c

+13 −2
Original line number Diff line number Diff line
@@ -2345,10 +2345,21 @@ 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.

	 */

	if (!IS_ENABLED(CONFIG_PREEMPT_RT))

		preempt_disable();

	read_unlock(&tasklist_lock);

	cgroup_enter_frozen();

	if (!IS_ENABLED(CONFIG_PREEMPT_RT))

		preempt_enable_no_resched();

	schedule();

	cgroup_leave_frozen(true);