[PATCH 4.9 63/77] s390/mm: fix race on mm->context.flush_mm

Greg Kroah-Hartman
2017-09-24T20:39:58Z

From: Martin Schwidefsky <schwidefsky@de.ibm.com>

commit 60f07c8ec5fae06c23e9fd7bab67dabce92b3414 upstream.

The order in __tlb_flush_mm_lazy is to flush TLB first and then clear
the mm->context.flush_mm bit. This can lead to missed flushes as the
bit can be set anytime, the order needs to be the other way aronud.

But this leads to a different race, __tlb_flush_mm_lazy may be called
on two CPUs concurrently. If mm->context.flush_mm is cleared first then
another CPU can bypass __tlb_flush_mm_lazy although the first CPU has
not done the flush yet. In a virtualized environment the time until the
flush is finally completed can be arbitrarily long.

Add a spinlock to serialize __tlb_flush_mm_lazy and use the function
in finish_arch_post_lock_switch as well.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/s390/include/asm/mmu.h | 2 ++
arch/s390/include/asm/mmu_context.h | 4 ++--
arch/s390/include/asm/tlbflush.h | 4 +++-
3 files changed, 7 insertions(+), 3 deletions(-)

--- a/arch/s390/include/asm/mmu.h
+++ b/arch/s390/include/asm/mmu.h
@@ -5,6 +5,7 @@
#include <linux/errno.h>

typedef struct {
+ spinlock_t lock;
cpumask_t cpu_attach_mask;
atomic_t flush_count;
unsigned int flush_mm;
@@ -25,6 +26,7 @@ typedef struct {
} mm_context_t;

#define INIT_MM_CONTEXT(name) \
+ .context.lock = __SPIN_LOCK_UNLOCKED(name.context.lock), \
.context.pgtable_lock = \
__SPIN_LOCK_UNLOCKED(name.context.pgtable_lock), \
.context.pgtable_list = LIST_HEAD_INIT(name.context.pgtable_list), \
--- a/arch/s390/include/asm/mmu_context.h
+++ b/arch/s390/include/asm/mmu_context.h
@@ -15,6 +15,7 @@
static inline int init_new_context(struct task_struct *tsk,
struct mm_struct *mm)
{
+ spin_lock_init(&mm->context.lock);
spin_lock_init(&mm->context.pgtable_lock);
INIT_LIST_HEAD(&mm->context.pgtable_list);
spin_lock_init(&mm->context.gmap_lock);
@@ -111,8 +112,7 @@ static inline void finish_arch_post_lock
while (atomic_read(&mm->context.flush_count))
cpu_relax();
cpumask_set_cpu(smp_processor_id(), mm_cpumask(mm));
- if (mm->context.flush_mm)
- __tlb_flush_mm(mm);
+ __tlb_flush_mm_lazy(mm);
preempt_enable();
}
set_fs(current->thread.mm_segment);
--- a/arch/s390/include/asm/tlbflush.h
+++ b/arch/s390/include/asm/tlbflush.h
@@ -96,10 +96,12 @@ static inline void __tlb_flush_kernel(vo

static inline void __tlb_flush_mm_lazy(struct mm_struct * mm)
{
+ spin_lock(&mm->context.lock);
if (mm->context.flush_mm) {
- __tlb_flush_mm(mm);
mm->context.flush_mm = 0;
+ __tlb_flush_mm(mm);
}
+ spin_unlock(&mm->context.lock);
}

/*