Messages in this thread Patch in this message | | | From | Thomas Gleixner <> | Subject | [GIT pull] core/core for v6.7-rc1 | Date | Sun, 29 Oct 2023 22:33:24 +0100 (CET) |
| |
Linus,
please pull the latest core/core branch from:
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core-core-2023-10-29-v2
up to: 1aabbc532413: signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT
Two small updates to ptrace_stop():
- Add a comment to explain that the preempt_disable() before unlocking tasklist lock is not a correctness problem and just avoids the tracer to preempt the tracee before the tracee schedules out.
- Make that preempt_disable() conditional on PREEMPT_RT=n.
RT enabled kernels cannot disable preemption at this point because cgroup_enter_frozen() and sched_submit_work() acquire spinlocks or rwlocks which are substituted by sleeping locks on RT. Acquiring a sleeping lock in a preemption disable region is obviously not possible.
This obviously brings back the potential slowdown of ptrace() for RT enabled kernels, but that's a price to be payed for latency guarantees.
Thanks,
tglx
------------------> Sebastian Andrzej Siewior (2): signal: Add a proper comment about preempt_disable() in ptrace_stop() signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT
kernel/signal.c | 33 ++++++++++++++++++++++++++++----- 1 file changed, 28 insertions(+), 5 deletions(-)
diff --git a/kernel/signal.c b/kernel/signal.c index 09019017d669..f2a5578326ad 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -2329,15 +2329,38 @@ static int ptrace_stop(int exit_code, int why, unsigned long message, do_notify_parent_cldstop(current, false, why); /* - * Don't want to allow preemption here, because - * sys_ptrace() needs this task to be inactive. + * The previous do_notify_parent_cldstop() invocation woke ptracer. + * One a PREEMPTION kernel this can result in preemption requirement + * which will be fulfilled after read_unlock() and the ptracer will be + * put on the CPU. + * The ptracer is in wait_task_inactive(, __TASK_TRACED) waiting for + * this task wait in schedule(). If this task gets preempted then it + * remains enqueued on the runqueue. The ptracer will observe this and + * then sleep for a delay of one HZ tick. In the meantime this task + * gets scheduled, enters schedule() and will wait for the ptracer. * - * XXX: implement read_unlock_no_resched(). + * This preemption point is not bad from a correctness point of + * view but extends the runtime by one HZ tick time due to the + * ptracer's sleep. The preempt-disable section ensures that there + * 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);
| |