Subject: Re: [PATCH] powerpc: mm: fixup preempt undefflow with huge pages

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 07 Mar 2016 21:04:05 +0530

Sebastian Andrzej Siewior <[email-masked]> writes:

> [ text/plain ]
> hugepd_free() used __get_cpu_var() once. Nothing ensured that the code
> accessing the variable did not migrate from one CPU to another and soon
> this was noticed by Tiejun Chen in 94b09d755462 ("powerpc/hugetlb:
> Replace __get_cpu_var with get_cpu_var"). So we had it fixed.
>
> Christoph Lameter was doing his __get_cpu_var() replaces and forgot
> PowerPC. Then he noticed this and sent his fixed up batch again which
> got applied as 69111bac42f5 ("powerpc: Replace __get_cpu_var uses").
>
> The careful reader will noticed one little detail: get_cpu_var() got
> replaced with this_cpu_ptr(). So now we have a put_cpu_var() which does
> a preempt_enable() and nothing that does preempt_disable() so we
> underflow the preempt counter.
>
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Cc: Christoph Lameter <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Tiejun Chen <[email-masked]>
> Cc: <[email-masked]>
> Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
> ---
>  arch/powerpc/mm/hugetlbpage.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index 744e24bcb85c..9e8919b39640 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -414,7 +414,7 @@ static void hugepd_free(struct mmu_gather *tlb, void *hugepte)
>  {
>  	struct hugepd_freelist **batchp;
>
> -	batchp = this_cpu_ptr(&hugepd_freelist_cur);
> +	batchp = get_cpu_ptr(&hugepd_freelist_cur);
>
>  	if (atomic_read(&tlb->mm->mm_users) < 2 ||
>  	    cpumask_equal(mm_cpumask(tlb->mm),

IMHO it would better if we do

batchp = get_cpu_var(hugepd_freelist_cpu);

so that it match the existing

put_cpu_var(hugepd_freelist_cur);

While you are there, can you also fix the wrong indentation on line 423
?

-aneesh