Messages in this thread | | | Date | Fri, 10 Feb 2023 18:08:47 +0100 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH v4] kernel/fork: beware of __put_task_struct calling context |
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On 2023-02-06 17:09:27 [-0800], Andrew Morton wrote: > On Mon, 6 Feb 2023 10:04:47 -0300 Wander Lairson Costa <wander@redhat.com> wrote: > > > Under PREEMPT_RT, __put_task_struct() indirectly acquires sleeping > > locks. Therefore, it can't be called from an non-preemptible context. > > Well that's regrettable. Especially if non-preempt kernels don't do > this.
Non-preemptible context on PREEMPT_RT. Interrupts handler and timers don't count as non-preemptible because interrupt handler are threaded and hrtimers are invoked in softirq context (which is preemptible on PREEMPT_RT).
This here is different because the hrtimer in question was marked as HRTIMER_MODE_REL_HARD. In this case it is invoked in hardirq context as requested with all the problems that follow.
> Why does PREEMPT_RT do this and can it be fixed?
PREEMPT_RT tries to move as much as it can out of hardirq context into preemptible context. A spinlock_t is preemptible on PREEMPT_RT while it is not in other preemption models. The scheduler needs to use raw_spinlock_t in order to be able to schedule a task from hardirq-context without a deadlock. For memory allocation only sleeping locks (spinlock_t) is used since there are no memory allocation/ deallocation on PREEMPT_RT in hardirq context. These two need to be separated.
> If it cannot be fixed then we should have a might_sleep() in > __put_task_struct() for all kernel configurations, along with an > apologetic comment explaining why.
__put_task_struct() should not be invoked in atomic context on PREEMPT_RT. It is fine however in a regular timer hrtimer. Adding might_sleep() will trigger a lot of false positives on a preemptible kernel and RT.
A might_lock() on a spinlock_t should do the trick from LOCKDEP perspective if CONFIG_PROVE_RAW_LOCK_NESTING is enabled. In this case it should be visible due to rq-lock or due to hrtimer.
Sebastian
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