Re: VFS: file-max limit 50044 reached

[Eric Dumazet <dada1@cosmosbay.com>]

Paul E. McKenney a écrit :
> > +/*
> > + *  Should we directly call rcu_do_batch() here ?
> > + *  if (unlikely(rdp->count > 10000))
> > + *      rcu_do_batch(rdp);
> > + */
>
>
> Good thing that the above is commented out!  ;-)
>
> Doing this can result in self-deadlock, for example with the following:
>
> 	spin_lock(&mylock);
> 	/* do some stuff. */
> 	call_rcu(&p->rcu_head, my_rcu_callback);
> 	/* do some more stuff. */
> 	spin_unlock(&mylock);
>
> void my_rcu_callback(struct rcu_head *p)
> {
> 	spin_lock(&mylock);
> 	/* self-deadlock via call_rcu() via rcu_do_batch()!!! */
> 	spin_unlock(&mylock);
> }
>
>
> 						Thanx, Paul

Thanks Paul for reminding us that call_rcu() should not ever call the callback 
function, as very well documented in Documentation/RCU/UP.txt
(Example 3: Death by Deadlock)

But is the same true for call_rcu_bh() ?

I intentionally wrote the comment to remind readers that a low maxbatch can 
trigger OOM in case a CPU is filled by some kind of DOS (network IRQ flood for 
example, targeting the IP dst cache)

To solve this problem, may be we could add a requirement to 
call_rcu_bh/callback functions  : If they have to lock a spinlock, only use a 
spin_trylock() and make them returns a status (0 : sucessfull callback, 1: 
please requeue me)

As most callback functions just kfree() some memory, most of OOM would be cleared.

int my_rcu_callback(struct rcu_head *p)
{
	if (!spin_trylock(&mylock))
		return 1; /* please call me later */
	/* do something here */
	...
	spin_unlock(&mylock);
	return 0;
}

(Changes to rcu_do_batch() are left as an exercice :) )

Eric
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