Re: [PATCH v17] exec: Fix dead-lock in de_thread with ptrace_attach

[Oleg Nesterov]

On 11/29, Bernd Edlinger wrote:
>
> On 11/23/25 19:32, Oleg Nesterov wrote:
> > I don't follow. Do you mean PREEMPT_RT ?
> >
> > If yes. In this case spin_lock_irq() is rt_spin_lock() which doesn't disable irqs,
> > it does rt_lock_lock() (takes rt_mutex) + migrate_disable().
> >
> > I do think that spin/mutex/whatever_unlock() is always safe. In any order, and
> > regardless of RT.
> >
>
> It is hard to follow how linux implements that spin_lock_irq exactly,

Yes ;)

> but
> to me it looks like it is done this way:
>
> include/linux/spinlock_api_smp.h:static inline void __raw_spin_lock_irq(raw_spinlock_t *lock)
> include/linux/spinlock_api_smp.h-{
> include/linux/spinlock_api_smp.h-       local_irq_disable();
> include/linux/spinlock_api_smp.h-       preempt_disable();
> include/linux/spinlock_api_smp.h-       spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
> include/linux/spinlock_api_smp.h-       LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);
> include/linux/spinlock_api_smp.h-}

Again, I will assume you mean RT.

In this case spinlock_t and raw_spinlock_t are not the same thing.

include/linux/spinlock_types.h:

	typedef struct spinlock {
		struct rt_mutex_base	lock;
	#ifdef CONFIG_DEBUG_LOCK_ALLOC
		struct lockdep_map	dep_map;
	#endif
	} spinlock_t;

include/linux/spinlock_rt.h:

	static __always_inline void spin_lock_irq(spinlock_t *lock)
	{
		rt_spin_lock(lock);
	}

rt_spin_lock() doesn't disable irqs, it takes "rt_mutex_base lock" and
disables migration.


> so an explicit task switch while locka_irq_disable looks
> very dangerous to me.

raw_spin_lock_irq() disables irqs/preemption regardless of RT, task switch
is not possible.

> Do you know other places where such
> a code pattern is used?

For example, double_lock_irq(). See task_numa_group(),

	double_lock_irq(&my_grp->lock, &grp->lock);

	....

	spin_unlock(&my_grp->lock);
	spin_unlock_irq(&grp->lock);

this can unlock the locks in reverse order.

I am sure there are more examples.

> I do just ask, because a close look at those might reveal
> some serious bugs, WDYT?

See above, I don't understand your concerns...

Oleg.