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Date: Thu, 13 Jul 2006 18:18:35 -0700
From: Andrew Morton <akpm@osdl.org>
To: Roland Dreier <rdreier@cisco.com>
Cc: arjan@infradead.org, mingo@elte.hu, zach.brown@oracle.com,
       openib-general@openib.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Convert idr's internal locking to _irqsave variant
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On Thu, 13 Jul 2006 18:08:17 -0700
Roland Dreier <rdreier@cisco.com> wrote:

>  > Good point, a try-again loop would work.  Do we really need the caller to
>  > maintain a cache?  I suspect something like
>  > 
>  > drat:
>  > 	if (idr_pre_get(GFP_KERNEL) == ENOMEM)
>  > 		give_up();
>  > 	spin_lock();
>  > 	ret = idr_get_new();
>  > 	spin_unlock();
>  > 	if (ret == ENOMEM)
>  > 		goto drat;
>  > 
>  > would do it.
> 
> The problem (for my tiny brain at least) is that I don't know where
> idr_pre_get() can put the memory it allocates if there's no lock in
> the idr structure -- how do you maintain internal consistency if no
> locks are held when filling the cache?

argh.  Aren't you supposed to be on vacation or something?

> Having the caller hold a chunk of memory in a stack variable was the
> trick I came up with to get around that.

Yes, that certainly works.