Re: [PATCH v4] kernel/fork: beware of __put_task_struct calling context

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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