linux-kernel - Re: [PATCH 22/25] Generic dynamic per cpu refcounting

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Message-ID: <20121129192003.GX16230@one.firstfloor.org>
Date:	Thu, 29 Nov 2012 20:20:03 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	linux-aio@...ck.org, linux-fsdevel@...r.kernel.org, zab@...hat.com,
	bcrl@...ck.org, jmoyer@...hat.com, axboe@...nel.dk,
	viro@...iv.linux.org.uk
Subject: Re: [PATCH 22/25] Generic dynamic per cpu refcounting

> The trick is that we don't watch for the refcount hitting 0 until we're
> shutting down - so this only works if you keep track of your initial
> refcount. As long as we're not shutting down, we know the refcount can't
> hit 0 because we haven't released the initial refcount.

This seems dangerous to me: assume you have one CPU which always
does get and another does put. So there may be 2^32 such operations
without a kill and you wrap for real in a way that does not get
corrected.

Normally this can only happen if you have a lot of objects or CPUs
are limited.

But you don't have any limit on getting out-of-sync. 

You could make it 64bit, but then wraps could happen.

-Andi
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