Re: Nasty regression from .27.7 to .27.8: idle samba goes crazy

[Manfred Spraul]

Chuck Ebbert wrote:
> On Mon, 8 Dec 2008 23:22:46 +0100
> Jan Rekorajski <baggins@xxxxxxxxxxxxxxxxx> wrote:
>
>   
> > On Mon, 08 Dec 2008, Stefan Richter wrote:
> >
> >     
> > > > > > On Monday, 8 of December 2008, Holger Hoffstaette wrote:
> > > > > >             
> > > > > > > The system is a generic old single-core P4 box with a single SATA drive,
> > > > > > > Gentoo userland and Samba is 3.0.33 (in async mode). The kernel has no
> > > > > > > patches or binary drivers.
> > > > > > >               
> > > Holger, it may be unrelated to the issue, but to be sure:  Which network
> > > card driver do you use?
> > >       
> > I think you can safely rule out NIC, I'm also seeing this behaviour on a
> > brand new server with imap hanging in some busy-loop.
> > Network card in my case:
> > Broadcom Corporation NetXtreme II BCM5708 Gigabit Ethernet (rev 12)
> >
> > What I observer was one CPU doing 100% system work, and the number of
> > timer interrupts went from 1k per second to 4k (for the whole system).
> >
> >     
Could you try the attached patch?
It should fix the bug.


--
   Manfred
rom ae060e0b7bc071bd73dd5319b93c3344d9e10212 Mon Sep 17 00:00:00 2001
From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
To: torvalds@xxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: cebbert@xxxxxxxxxx
Cc: airlied@xxxxxxxxx
Cc: akpm@xxxxxxxxxxxxxxxxxxxx
Bcc: manfred@xxxxxxxxxxxxxxxx
Date: Wed, 10 Dec 2008 18:17:06 +0100
Subject: [PATCH] lib/idr.c: Fix bug introduced by RCU fix

The last patch to lib/idr.c caused a bug if idr_get_new_above() was
called on an empty idr:
Usually, nodes stay on the same layer. New layers are added to the top
of the tree.
The exception is idr_get_new_above() on an empty tree: In this case,
the new root node is first added on layer 0, then moved upwards.
p->layer was not updated.

As usual: You shall never rely on the source code comments, they
will only mislead you.

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
---
 lib/idr.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/lib/idr.c b/lib/idr.c
index 7a785a0..1c4f928 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -220,8 +220,14 @@ build_up:
 	 */
 	while ((layers < (MAX_LEVEL - 1)) && (id >= (1 << (layers*IDR_BITS)))) {
 		layers++;
-		if (!p->count)
+		if (!p->count) {
+			/* special case: if the tree is currently empty,
+			 * then we grow the tree by moving the top node
+			 * upwards.
+			 */
+			p->layer++;
 			continue;
+		}
 		if (!(new = get_from_free_list(idp))) {
 			/*
 			 * The allocation failed.  If we built part of
-- 
1.5.6.5