Subject: Re: [RFC][PATCH 3/4] deadlock prevention core

From: Peter Zijlstra <[email-masked]>
Date: Sat, 12 Aug 2006 20:47:11 +0200

On Sat, 2006-08-12 at 20:32 +0200, Indan Zupancic wrote:
> On Sat, August 12, 2006 20:08, Peter Zijlstra said:
> > On Sat, 2006-08-12 at 19:54 +0200, Indan Zupancic wrote:
> >> True, but currently memalloc_reserve isn't used in a sensible way,
> >> or I'm missing something.
> >
> > Well, I'm somewhat reluctant to stick network related code into mm/, it
> > seems well separated now.
> 
> What I had in mind was something like:
> 
> +static DEFINE_SPINLOCK(memalloc_lock);
> +static int memalloc_socks;
> +
> +atomic_t memalloc_skbs_used;
> +EXPORT_SYMBOL_GPL(memalloc_skbs_used);
> +
> +int sk_adjust_memalloc(int nr_socks)
> +{
> +	unsigned long flags;
> +	unsigned int reserve;
> +	int err;
> +
> +	spin_lock_irqsave(&memalloc_lock, flags);
> +
> +	memalloc_socks += nr_socks;
> +	BUG_ON(memalloc_socks < 0);
> +
> +	reserve = nr_socks * (2 * MAX_PHYS_SEGMENTS + 	/* outbound */
> +			      5 * MAX_CONCURRENT_SKBS);	/* inbound */
> +
> +	err = adjust_memalloc_reserve(reserve);
> +	spin_unlock_irqrestore(&memalloc_lock, flags);
> +	if (err) {
> +		printk(KERN_WARNING
> +			"Unable to change RX reserve to: %lu, error: %d\n",
> +			reserve, err);
> +	}
> +	return err;
> +}
> 
> The original code missed the brackets, so 5 * MAX_CONCURRENT_SKBS wasn't done
> per socket. But the comment said it was per socket, so I added in this version.

Ah right, I did that in v3, with a similar comment, but I realised that
the inbound reserve need not be per socket, and that the comment was
ambiguous enough to allow this reading.

Why not per socket, its used to place an upper bound, its not
calculating one, its setting one.

Like you can see in memalloc_skbs_inc(), it limits to
MAX_CONCURRENT_SKBS.