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From: Linus Torvalds <torvalds@osdl.org>
Subject: Re: [ANNOUNCE] CGit v0.1-pre
Date: Sun, 10 Dec 2006 17:04:15 -0800 (PST)
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To: Lars Hjemli <hjemli@gmail.com>
Sender: git-owner@vger.kernel.org



On Mon, 11 Dec 2006, Lars Hjemli wrote:
> 
> For a quick summary of the cache algorithm, the project README is here:
> 
>  http://hjemli.net/git/cgit/view/?id=5917c37ce30b3f0a374c9fa376955f51f1d7bfbf

Your pseudo-algorithm is dubious:

		name = generate_cache_name(request);
	top:
		if (!exists(name)) {
			if (lock_cache(name)) {
				generate_cache(request, name);
				unlock_cache(name);
			} else {
				sched_yield();
				goto top;
			}
		} else if (expired(name)) {
			if (lock_cache(name)) {
				generate_cache(request, name);
				unlock_cache(name);
			}
		}
		print_file(name);


You really should have:

	if (!exists) {
		if (!lock)
			delay-and-repeat;
		/* RETEST exists _after_ getting the lock */
		if (!exists) {
			generate into lock-file
			mv lockfile exists;
		} else {
			rm lockfile
		}
	}

because you really want to re-check the existence after you got the lock, 
otherwise you would race with somebody else that got the lock, generated 
the data, and then unlocked (and you got the lock _after_ the data was 
generated, so now you generate it unnecessarily).

As a side note: how do you release your caches?