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);
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