kernel/locking/rwsem.h

   1 /*
   2  * The owner field of the rw_semaphore structure will be set to
   3  * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
   4  * the owner field when it unlocks. A reader, on the other hand, will
   5  * not touch the owner field when it unlocks.
   6  *
   7  * In essence, the owner field now has the following 3 states:
   8  *  1) 0
   9  *     - lock is free or the owner hasn't set the field yet
  10  *  2) RWSEM_READER_OWNED
  11  *     - lock is currently or previously owned by readers (lock is free
  12  *       or not set by owner yet)
  13  *  3) Other non-zero value
  14  *     - a writer owns the lock
  15  */
  16 #define RWSEM_READER_OWNED      ((struct task_struct *)1UL)
  17
  18 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  19 /*
  20  * All writes to owner are protected by WRITE_ONCE() to make sure that
  21  * store tearing can't happen as optimistic spinners may read and use
  22  * the owner value concurrently without lock. Read from owner, however,
  23  * may not need READ_ONCE() as long as the pointer value is only used
  24  * for comparison and isn't being dereferenced.
  25  */
  26 static inline void rwsem_set_owner(struct rw_semaphore *sem)
  27 {
  28         WRITE_ONCE(sem->owner, current);
  29 }
  30
  31 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
  32 {
  33         WRITE_ONCE(sem->owner, NULL);
  34 }
  35
  36 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
  37 {
  38         /*
  39          * We check the owner value first to make sure that we will only
  40          * do a write to the rwsem cacheline when it is really necessary
  41          * to minimize cacheline contention.
  42          */
  43         if (sem->owner != RWSEM_READER_OWNED)
  44                 WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
  45 }
  46
  47 static inline bool rwsem_owner_is_writer(struct task_struct *owner)
  48 {
  49         return owner && owner != RWSEM_READER_OWNED;
  50 }
  51
  52 static inline bool rwsem_owner_is_reader(struct task_struct *owner)
  53 {
  54         return owner == RWSEM_READER_OWNED;
  55 }
  56 #else
  57 static inline void rwsem_set_owner(struct rw_semaphore *sem)
  58 {
  59 }
  60
  61 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
  62 {
  63 }
  64
  65 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
  66 {
  67 }
  68 #endif