[PATCH-tip v7 20/20] locking/rwsem: Disable preemption in down_read*() if owner in count

[Waiman Long]

It is very unlikely that successive preemption at the middle of
down_read's inc-check-dec sequence will cause the reader count to
overflow, For absolute correctness, however, we still need to prevent
that possibility from happening. So preemption will be disabled during
the down_read*() call.

For PREEMPT=n kernels, there isn't much overhead in doing that.
For PREEMPT=y kernels, there will be some additional cost. RT kernels
have their own rwsem code, so it will not be a problem for them.

If MERGE_OWNER_INTO_COUNT isn't defined, we don't need to worry about
reader count overflow and so we don't need to disable preemption.

Signed-off-by: Waiman Long <long...@redhat.com>
---
 kernel/locking/rwsem.c | 39 +++++++++++++++++++++++++++++++++++----
 1 file changed, 35 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index cd7fdb8b02ed..60783267b50d 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -375,6 +375,24 @@ static inline void rwsem_set_nonspinnable(struct 
rw_semaphore *sem)
 }
 
 #ifdef MERGE_OWNER_INTO_COUNT
+/*
+ * It is very unlikely that successive preemption at the middle of
+ * down_read's inc-check-dec sequence will cause the reader count to
+ * overflow, For absolute correctness, we still need to prevent
+ * that possibility from happening. So preemption will be disabled
+ * during the down_read*() call.
+ *
+ * For PREEMPT=n kernels, there isn't much overhead in doing that.
+ * For PREEMPT=y kernels, there will be some additional cost.
+ *
+ * If MERGE_OWNER_INTO_COUNT isn't defined, we don't need to worry
+ * about reader count overflow and so we don't need to disable
+ * preemption.
+ */
+#define rwsem_preempt_disable()                        preempt_disable()
+#define rwsem_preempt_enable()                 preempt_enable()
+#define rwsem_schedule_preempt_disabled()      schedule_preempt_disabled()
+
 /*
  * Get the owner value from count to have early access to the task structure.
  * Owner from sem->count should includes the RWSEM_NONSPINNABLE bits
@@ -431,6 +449,11 @@ static int __init rwsem_show_count_status(void)
 }
 late_initcall(rwsem_show_count_status);
 #else /* !MERGE_OWNER_INTO_COUNT */
+
+#define rwsem_preempt_disable()
+#define rwsem_preempt_enable()
+#define rwsem_schedule_preempt_disabled()      schedule()
+
 static inline struct task_struct *rwsem_get_owner(struct rw_semaphore *sem)
 {
        return READ_ONCE(sem->owner);
@@ -1255,7 +1278,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int 
state, long adjustment)
                        raw_spin_unlock_irq(&sem->wait_lock);
                        break;
                }
-               schedule();
+               rwsem_schedule_preempt_disabled();
                lockevent_inc(rwsem_sleep_reader);
        }
 
@@ -1486,28 +1509,36 @@ static struct rw_semaphore *rwsem_downgrade_wake(struct 
rw_semaphore *sem)
  */
 inline void __down_read(struct rw_semaphore *sem)
 {
-       long tmp, adjustment = rwsem_read_trylock(sem, &tmp);
+       long tmp, adjustment;
 
+       rwsem_preempt_disable();
+       adjustment = rwsem_read_trylock(sem, &tmp);
        if (unlikely(tmp & RWSEM_READ_FAILED_MASK)) {
                rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, adjustment);
                DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
        } else {
                rwsem_set_reader_owned(sem);
        }
+       rwsem_preempt_enable();
 }
 
 static inline int __down_read_killable(struct rw_semaphore *sem)
 {
-       long tmp, adjustment = rwsem_read_trylock(sem, &tmp);
+       long tmp, adjustment;
 
+       rwsem_preempt_disable();
+       adjustment = rwsem_read_trylock(sem, &tmp);
        if (unlikely(tmp & RWSEM_READ_FAILED_MASK)) {
                if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE,
-                                                   adjustment)))
+                                                   adjustment))) {
+                       rwsem_preempt_enable();
                        return -EINTR;
+               }
                DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
        } else {
                rwsem_set_reader_owned(sem);
        }
+       rwsem_preempt_enable();
        return 0;
 }
 
-- 
2.18.1