Re: [PATCH 2/6] idr: remove MAX_IDR_MASK and move left MAX_IDR_* into idr.c

[Tejun Heo <tj@kernel.org>]

Hello,

On Fri, Feb 08, 2013 at 10:09:13PM +0000, Hefty, Sean wrote:
> >   Used to wrap cyclic @start.  Can be replaced with max(next, 0).
> >   Note that this type of cyclic allocation using idr is buggy.  These
> >   are prone to spurious -ENOSPC failure after the first wraparound.
> 
> The replacement code looks fine, but can you explain why the use is buggy?

So, if you want a cyclic allocation, the allocation should be tried in
[start, END) and then [0, start); otherwise, after the allocation
wraps for the first time, as the closer the starting point gets to
END, the chance of not finding a vacant slot in [start, END) goes
higher.  When @start equals END - 1 for the second time, if the first
END - 1 allocation is still around, you'll get -ENOSPC.

In practice, I don't think anyone is hitting this.  idr has always
been horribly broken when it reaches higher range (> 1<<30 on 64bit)
so things would have broken even before the first wraparound.  It
still is a theoretical possibility which may trigger if idr is used
for, say, ipc messages or storage commands.

Thanks.

-- 
tejun