From 3c62ecdaf1606ef4cbcb116f3c8107ddbbf39b51 Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Date: Thu, 3 Aug 2023 12:09:32 +0200 Subject: [PATCH 009/204] 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@redhat.com> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Link: https://lore.kernel.org/r/20230803100932.325870-3-bigeasy@linutronix.de --- kernel/signal.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) Index: linux-6.6.58-rt45/kernel/signal.c =================================================================== @ linux-6.6.58-rt45/kernel/signal.c:2348 @ 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);