powerpc/mm: Fix a AB->BA deadlock scenario with nohash MMU context lock

The MMU context_lock can be taken from switch_mm() while the
rq->lock is held. The rq->lock can also be taken from interrupts,
thus if we get interrupted in destroy_context() with the context
lock held and that interrupt tries to take the rq->lock, there's
a possible deadlock scenario with another CPU having the rq->lock
and calling switch_mm() which takes our context lock.

The fix is to always ensure interrupts are off when taking our
context lock. The switch_mm() path is already good so this fixes
the destroy_context() path.

While at it, turn the context lock into a new style spinlock.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/arch/powerpc/mm/mmu_context_nohash.c b/arch/powerpc/mm/mmu_context_nohash.c
index c42858780cbd..8343986809c0 100644
--- a/arch/powerpc/mm/mmu_context_nohash.c
+++ b/arch/powerpc/mm/mmu_context_nohash.c
@@ -46,7 +46,7 @@ static unsigned int next_context, nr_free_contexts;
 static unsigned long *context_map;
 static unsigned long *stale_map[NR_CPUS];
 static struct mm_struct **context_mm;
-static spinlock_t context_lock = SPIN_LOCK_UNLOCKED;
+static DEFINE_SPINLOCK(context_lock);
 
 #define CTX_MAP_SIZE    \
         (sizeof(unsigned long) * (last_context / BITS_PER_LONG + 1))
@@ -276,6 +276,7 @@ int init_new_context(struct task_struct *t, struct mm_struct *mm)
  */
 void destroy_context(struct mm_struct *mm)
 {
+        unsigned long flags;
         unsigned int id;
 
         if (mm->context.id == MMU_NO_CONTEXT)
@@ -283,7 +284,7 @@ void destroy_context(struct mm_struct *mm)
 
         WARN_ON(mm->context.active != 0);
 
-        spin_lock(&context_lock);
+        spin_lock_irqsave(&context_lock, flags);
         id = mm->context.id;
         if (id != MMU_NO_CONTEXT) {
                 __clear_bit(id, context_map);
@@ -294,7 +295,7 @@ void destroy_context(struct mm_struct *mm)
                 context_mm[id] = NULL;
                 nr_free_contexts++;
         }
-        spin_unlock(&context_lock);
+        spin_unlock_irqrestore(&context_lock, flags);
 }
 
 #ifdef CONFIG_SMP