[PATCH] percpu: don't assume existence of cpu0

From: Tejun Heo
Date: Tue Sep 01 2009 - 08:31:04 EST

percpu incorrectly assumed that cpu0 was always there which led to the
following warning and eventual oops on sparc machines w/o cpu0.

WARNING: at mm/percpu.c:651 pcpu_map+0xdc/0x100()
Modules linked in:
Call Trace:
[000000000045eb70] warn_slowpath_common+0x50/0xa0
[000000000045ebdc] warn_slowpath_null+0x1c/0x40
[00000000004d493c] pcpu_map+0xdc/0x100
[00000000004d59a4] pcpu_alloc+0x3e4/0x4e0
[00000000004d5af8] __alloc_percpu+0x18/0x40
[00000000005b112c] __percpu_counter_init+0x4c/0xc0
Unable to handle kernel NULL pointer dereference
I7: <sysfs_new_dirent+0x30/0x120>
Disabling lock debugging due to kernel taint
Caller[000000000053c1b0]: sysfs_new_dirent+0x30/0x120
Caller[000000000053c7a4]: create_dir+0x24/0xc0
Caller[000000000053c870]: sysfs_create_dir+0x30/0x80
Caller[00000000005990e8]: kobject_add_internal+0xc8/0x200
Kernel panic - not syncing: Attempted to kill the idle task!

This patch fixes the problem by backporting parts from devel branch to
make percpu core not depend on the existence of cpu0.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Meelis Roos <mroos@xxxxxxxx>
Cc: David Miller <davem@xxxxxxxxxxxxx>
Meelis Roos wrote:
>> Aha... Does the following patch fix the problem?
> Yes, yesterdays 2.6.31-rc8-git plus this patch seems to work fine, no
> warnings/oopses/panics. It is happily churning on debian unstable
> upgrade now.

Good to hear. Will forward to Linus right away. Thanks.

mm/percpu.c | 15 ++++++++++++++-
1 files changed, 14 insertions(+), 1 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 5fe3784..3311c89 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -197,7 +197,12 @@ static unsigned long pcpu_chunk_addr(struct pcpu_chunk *chunk,
static bool pcpu_chunk_page_occupied(struct pcpu_chunk *chunk,
int page_idx)
- return *pcpu_chunk_pagep(chunk, 0, page_idx) != NULL;
+ /*
+ * Any possible cpu id can be used here, so there's no need to
+ * worry about preemption or cpu hotplug.
+ */
+ return *pcpu_chunk_pagep(chunk, raw_smp_processor_id(),
+ page_idx) != NULL;

/* set the pointer to a chunk in a page struct */
@@ -297,6 +302,14 @@ static struct pcpu_chunk *pcpu_chunk_addr_search(void *addr)
return pcpu_first_chunk;

+ /*
+ * The address is relative to unit0 which might be unused and
+ * thus unmapped. Offset the address to the unit space of the
+ * current processor before looking it up in the vmalloc
+ * space. Note that any possible cpu id can be used here, so
+ * there's no need to worry about preemption or cpu hotplug.
+ */
+ addr += raw_smp_processor_id() * pcpu_unit_size;
return pcpu_get_page_chunk(vmalloc_to_page(addr));


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