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
>
>