Oh well yes, this doesnt quite work on !SMP.

And this kind of bug is frequent....

See following example :

commit b9980cdcf2524c5fe15d8cbae9c97b3ed6385563

Author: Hugh Dickins <hughd@google.com>

Date:   Wed Feb 8 17:13:40 2012 -0800

    mm: fix UP THP spin_is_locked BUGs

    

    Fix CONFIG_TRANSPARENT_HUGEPAGE=y CONFIG_SMP=n CONFIG_DEBUG_VM=y

    CONFIG_DEBUG_SPINLOCK=n kernel: spin_is_locked() is then always false,

    and so triggers some BUGs in Transparent HugePage codepaths.

    

    asm-generic/bug.h mentions this problem, and provides a WARN_ON_SMP(x);

    but being too lazy to add VM_BUG_ON_SMP, BUG_ON_SMP, WARN_ON_SMP_ONCE,

    VM_WARN_ON_SMP_ONCE, just test NR_CPUS != 1 in the existing VM_BUG_ONs.

    

    Signed-off-by: Hugh Dickins <hughd@google.com>

    Cc: Andrea Arcangeli <aarcange@redhat.com>

    Cc: <stable@vger.kernel.org>

    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c

index b3ffc21..91d3efb 100644

--- a/mm/huge_memory.c

+++ b/mm/huge_memory.c

@@ -2083,7 +2083,7 @@ static void collect_mm_slot(struct mm_slot *mm_slot)

 {

        struct mm_struct *mm = mm_slot->mm;

 

-       VM_BUG_ON(!spin_is_locked(&khugepaged_mm_lock));

+       VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&khugepaged_mm_lock));

On Sun, Jan 13, 2013 at 1:39 PM, Felix Fietkau <nbd@openwrt.org> wrote:

On 2013-01-13 7:03 PM, Eric Dumazet wrote:
> I suspect a bug in the spin_is_locked() implementation on your arch, as
> he socket lock should be held at this point.

I don't think this is an arch implementation bug, this probably happens
on all !SMP systems. See this bit from include/linux/spinlock_up.h:

#define arch_spin_is_locked(lock)   ((void)(lock), 0)

- Felix