Re: [tip:locking/core] locking/rwsem: Fix lock optimistic spinning when owner is not running

[Oleg Nesterov]

I think the patch is fine, but this reminds me...

On 03/07, tip-bot for Jason Low wrote:
>
>  bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
>  {
>  	long count;
>
>  	rcu_read_lock();
> -	while (owner_running(sem, owner)) {
> -		/* abort spinning when need_resched */
> -		if (need_resched()) {
> +	while (sem->owner == owner) {
> +		/*
> +		 * Ensure we emit the owner->on_cpu, dereference _after_
> +		 * checking sem->owner still matches owner, if that fails,
> +		 * owner might point to free()d memory, if it still matches,
> +		 * the rcu_read_lock() ensures the memory stays valid.
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Yes, this is another case when we wrongly assume this.

Peter, should I resend

	[PATCH 3/3] introduce task_rcu_dereference()
	http://marc.info/?l=linux-kernel&m=141443631413914

? or should we add another call_rcu() in finish_task_switch() (like -rt does)
to make this true?

Oleg.

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