linux-kernel - Re: [PATCH] [1/2] SYSCTL: Make bin

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Date:	Sat, 19 Dec 2009 09:40:56 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>, ebiederm@...ssion.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [1/2] SYSCTL: Make bin_table not const



On Sat, 19 Dec 2009, Andi Kleen wrote:
> 
> Turn all the bin_tables from const to non-const. This is needed for the next
> patch. Unfortunately there were a lot of them, so the patch is rather 
> large, but it is completely mechanic. 

Ouch. I hate this.

I'd much rather have a constant sysctl bin_table.

If the _only_ thing you want to do is to do a "warn-on-once" logic, I 
would seriously suggest you use a small hash-table instead of turning 
bin_table non-const.

IOW, d something like this:

	#define WARN_ONCE_HASH_BITS (10)
	#define WARN_ONCE_HASH_SIZE (1<<WARN_ONCE_HASH_BITS)

	static DECLARE_BITMAP(warn_once_bitmap, WARN_ONCE_HASH_SIZE);

	/*
	 * NOTE! We don't use the SMP-safe bit tests. We simply
	 * don't care enough. 
	 */
	static void warn_on_bintable(const int *name, int nlen)
	{
		unsigned int hash = full_name_hash(name, nlen);
		hash = hash_32(hash, WARN_ONCE_HASH_BITS);
		if (__test_and_set_bit(warn_once_bitmap, hash))
			return;
		deprecated_sysctl_warning(name, nlen);
	}

which should do the right thing in all practical circumstances.

(The above is obviously totally untested, I wrote it in the mail reader, 
but it's obvious enough what the intent is).

			Linus
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