linux-kernel - idr_get_new_above() limitation?

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Date:	Mon, 2 Jul 2007 19:19:26 +0200
From:	Hoang-Nam Nguyen <hnguyen@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	openib-general@...nib.org, jim.houston@...r.com
Cc:	Stefan Roscher <ossrosch@...ux.vnet.ibm.com>, raisch@...ibm.com
Subject: idr_get_new_above() limitation?

Hello,
For ehca device driver we're intending to utilize 
idr_get_new_above() and have written a test case, which I'm attaching
at the end. Basically it tries to get an idr token above a lower boundary
by calling idr_get_new_above() and then uses idr_find() to check if 
the returned token can be found. 
Here is our observation with 2.6.22-rc7 on ppc64:

Use lower boundary 0x3ffffffc
[root@xyz idr_bug]# insmod idr_test_mod.ko start=1073741820
insmod: error inserting 'idr_test_mod.ko': -1 Unknown symbol in module
[root@xyz idr_bug]# dmesg -c
i=3ffffffc token=3ffffffc t=000000003ffffffc
i=3ffffffd token=3ffffffd t=000000003ffffffd
i=3ffffffe token=3ffffffe t=000000003ffffffe
i=3fffffff token=3fffffff t=000000003fffffff
i=40000000 token=40000000 t=0000000000000000
Invalid object 0000000000000000. Expected 40000000

That means token 0x40000000 seems to be the "upper boundary" of idr_find().
However the behaviour is not consistent in that it was returned by
idr_get_new_above().

Looking at void *idr_find(struct idr *idp, int id)
{
	int n;
	struct idr_layer *p;

	n = idp->layers * IDR_BITS;
	p = idp->top;

	/* Mask off upper bits we don't use for the search. */
	id &= MAX_ID_MASK;

	if (id >= (1 << n))
		return NULL;

	while (n > 0 && p) {
		n -= IDR_BITS;
		p = p->ary[(id >> n) & IDR_MASK];
	}
	return((void *)p);
}
we found that the if-condition has failed:
  layers = 5
  IDR_BITS = 6
  n = 30
  (id >= (1 << n)) = (0x40000000 >= 0x40000000) = 1

Since MAX_ID_MASK=0x7fffffff, I'm wondering if 0x40000000 is the actual
upper boundary. Any hints or suggestions are appreciated.

Thanks!
Nam

#include <linux/module.h>
#include <linux/idr.h>

MODULE_LICENSE("GPL");

int start_opt = 0x7e000000;

module_param_named(start, start_opt, int, 0);

MODULE_PARM_DESC(start,
		 "Start token for idr_get_new_above(). Default 0x7e000000");

static int __init idr_test_init(void)
{
	DEFINE_IDR(idr);
	int token, ret;
	unsigned long i;

	for (i = start_opt;  i <= MAX_ID_MASK; i++) {
		void * t;
		if (!idr_pre_get(&idr, GFP_KERNEL)) {
			printk(KERN_ERR "ERROR: Out of mem\n");
			return -ENOENT;
		}
		ret = idr_get_new_above(&idr, (void*)i, start_opt, &token);
		switch (ret) {
		case 0:
			t = idr_find(&idr, token);
			printk(KERN_ERR "i=%lx token=%x t=%p\n", i, token, t);
			if (t != (void*)i) {
				printk(KERN_ERR "Invalid object %p. Expected %lx\n",
				       t, i);
				return -ENOENT;
			}
			break;
		case -EAGAIN:
			i--;
			printk("idr_get_new_above() ret=-EAGAIN\n");
			break;
		default:
			printk(KERN_ERR "ERROR: Out of mem\n");
			break;
		}
	}
	/*
	 * return an error in any case since we don't need the module
	 * loaded anyway.
	 */
	return -ENOENT;
}

static void __exit idr_test_exit(void)
{
	printk(KERN_ERR "module exit\n");
}

module_init(idr_test_init);
module_exit(idr_test_exit);

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