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To: Andrew Morton <akpm@osdl.org>
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Subject: Re: [PATCH] Convert idr's internal locking to _irqsave variant
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From: Roland Dreier <rdreier@cisco.com>
Date: Thu, 13 Jul 2006 18:08:17 -0700
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 > 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?

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

 - R.