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

[Peter Zijlstra]

On Sat, Mar 07, 2015 at 06:13:47PM +0100, Oleg Nesterov wrote:
> 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?

Yeah, I think the extra call_rcu() makes most sense.
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