Re: VFS: file-max limit 50044 reached

[Paul E. McKenney]

On Tue, Oct 18, 2005 at 11:46:30AM +0200, Eric Dumazet wrote:
> 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() ?

Yes, same rules for this aspect of call_rcu_bh() and call_rcu().

> 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 :) )

Another approach that would keep the current easier-to-use semantics
would be to schedule a tasklet or workqueue to process the callbacks
in a safe context.

						Thanx, Paul
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