Re: [PATCH v2] powerpc: mm: fixup preempt undefflow with huge pages

[Sebastian Andrzej Siewior <sebastian@breakpoint.cc>]

On 2016-03-10 01:04:24 [+0530], Aneesh Kumar K.V wrote:
> Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:

*ping*
http://patchwork.ozlabs.org/patch/593943/

> > [ 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.
> >
>=20
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>=20
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> > v1=E2=80=A6v2: - use get_cpu_var() instead of get_cpu_ptr()
> >        - correct indentation of put_cpu_var()
> >
> >  arch/powerpc/mm/hugetlbpage.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpag=
e.c
> > index 744e24bcb85c..4a811ca7ac9d 100644
> > --- a/arch/powerpc/mm/hugetlbpage.c
> > +++ b/arch/powerpc/mm/hugetlbpage.c
> > @@ -414,13 +414,13 @@ static void hugepd_free(struct mmu_gather *tlb, v=
oid *hugepte)
> >  {
> >  	struct hugepd_freelist **batchp;
> > =20
> > -	batchp =3D this_cpu_ptr(&hugepd_freelist_cur);
> > +	batchp =3D &get_cpu_var(hugepd_freelist_cur);
> > =20
> >  	if (atomic_read(&tlb->mm->mm_users) < 2 ||
> >  	    cpumask_equal(mm_cpumask(tlb->mm),
> >  			  cpumask_of(smp_processor_id()))) {
> >  		kmem_cache_free(hugepte_cache, hugepte);
> > -        put_cpu_var(hugepd_freelist_cur);
> > +		put_cpu_var(hugepd_freelist_cur);
> >  		return;
> >  	}
> > =20

Sebastian