[PATCH] idr: fix a critical misallocation bug, take#2

[Tejun Heo <tj@kernel.org>]

This is retry of reverted 859ddf09743a8cc680af33f7259ccd0fd36bfe9d
which contained two bugs.

* pa[idp->layers] should be cleared even if it's not used by
  sub_alloc() because it's used by mark idr_mark_full().

* The original condition check also assigned pa[l] to p which the new
  code didn't do thus leaving p pointing at the wrong layer.

Both problems have been fixed and the idr code has received good
amount testing using userland testing setup where simple bitmap
allocator is run parallel to verify the result of idr allocation.

The bug this patch fixes is caused by sub_alloc() optimization path
bypassing out-of-room condition check and restarting allocation loop
with starting value higher than maximum allowed value.  For detailed
description, please read commit message of 859ddf09.

Signed-off-by: Tejun Heo <tj@kernel.org>
Based-on-patch-from: Eric Paris <eparis@redhat.com>
Reported-by: Eric Paris <eparis@redhat.com>
Tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
---
Heh... Embarrassingly, it turns out Eric's original patch is correct
and all I did was adding two mistakes. :-) I'm pretty sure this is the
correct fix and have tested it quite extensively.  But, given the
fragility of this thing, Andrew, can you please put it in your tree
and push it after 2.6.33 merge window opens?  Greg, please don't put
this into -stable until at least 2.6.33-rc2 seems okay.

Thank you.

 lib/idr.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/idr.c b/lib/idr.c
index 1cac726..0dc7822 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -156,10 +156,12 @@ static int sub_alloc(struct idr *idp, int *starting_id, struct idr_layer **pa)
 			id = (id | ((1 << (IDR_BITS * l)) - 1)) + 1;
 
 			/* if already at the top layer, we need to grow */
-			if (!(p = pa[l])) {
+			if (id >= 1 << (idp->layers * IDR_BITS)) {
 				*starting_id = id;
 				return IDR_NEED_TO_GROW;
 			}
+			p = pa[l];
+			BUG_ON(!p);
 
 			/* If we need to go up one layer, continue the
 			 * loop; otherwise, restart from the top.