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signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT

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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@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20230803100932.325870-3-bigeasy@linutronix.de
This commit is contained in:
Sebastian Andrzej Siewior 2023-08-03 12:09:32 +02:00 committed by Thomas Gleixner
parent a20d6f63db
commit 1aabbc5324
1 changed files with 13 additions and 2 deletions
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15
kernel/signal.c
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@ -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);