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-rw-r--r--kernel/locking/rwsem.h52
1 files changed, 50 insertions, 2 deletions
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index 870ed9a5b426..a699f4048ba1 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -1,14 +1,58 @@
+/*
+ * The owner field of the rw_semaphore structure will be set to
+ * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
+ * the owner field when it unlocks. A reader, on the other hand, will
+ * not touch the owner field when it unlocks.
+ *
+ * In essence, the owner field now has the following 3 states:
+ * 1) 0
+ * - lock is free or the owner hasn't set the field yet
+ * 2) RWSEM_READER_OWNED
+ * - lock is currently or previously owned by readers (lock is free
+ * or not set by owner yet)
+ * 3) Other non-zero value
+ * - a writer owns the lock
+ */
+#define RWSEM_READER_OWNED ((struct task_struct *)1UL)
+
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+/*
+ * All writes to owner are protected by WRITE_ONCE() to make sure that
+ * store tearing can't happen as optimistic spinners may read and use
+ * the owner value concurrently without lock. Read from owner, however,
+ * may not need READ_ONCE() as long as the pointer value is only used
+ * for comparison and isn't being dereferenced.
+ */
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
- sem->owner = current;
+ WRITE_ONCE(sem->owner, current);
}
static inline void rwsem_clear_owner(struct rw_semaphore *sem)
{
- sem->owner = NULL;
+ WRITE_ONCE(sem->owner, NULL);
+}
+
+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+ /*
+ * We check the owner value first to make sure that we will only
+ * do a write to the rwsem cacheline when it is really necessary
+ * to minimize cacheline contention.
+ */
+ if (sem->owner != RWSEM_READER_OWNED)
+ WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
+}
+
+static inline bool rwsem_owner_is_writer(struct task_struct *owner)
+{
+ return owner && owner != RWSEM_READER_OWNED;
}
+static inline bool rwsem_owner_is_reader(struct task_struct *owner)
+{
+ return owner == RWSEM_READER_OWNED;
+}
#else
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
@@ -17,4 +61,8 @@ static inline void rwsem_set_owner(struct rw_semaphore *sem)
static inline void rwsem_clear_owner(struct rw_semaphore *sem)
{
}
+
+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+}
#endif