kernel/locking/rwsem.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 /*
   3  * The owner field of the rw_semaphore structure will be set to
   4  * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
   5  * the owner field when it unlocks. A reader, on the other hand, will
   6  * not touch the owner field when it unlocks.
   7  *
   8  * In essence, the owner field now has the following 3 states:
   9  *  1) 0
  10  *     - lock is free or the owner hasn't set the field yet
  11  *  2) RWSEM_READER_OWNED
  12  *     - lock is currently or previously owned by readers (lock is free
  13  *       or not set by owner yet)
  14  *  3) Other non-zero value
  15  *     - a writer owns the lock
  16  */
  17 #define RWSEM_READER_OWNED      ((struct task_struct *)1UL)
  18
  19 #ifdef CONFIG_DEBUG_RWSEMS
  20 # define DEBUG_RWSEMS_WARN_ON(c)        DEBUG_LOCKS_WARN_ON(c)
  21 #else
  22 # define DEBUG_RWSEMS_WARN_ON(c)
  23 #endif
  24
  25 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  26 /*
  27  * All writes to owner are protected by WRITE_ONCE() to make sure that
  28  * store tearing can't happen as optimistic spinners may read and use
  29  * the owner value concurrently without lock. Read from owner, however,
  30  * may not need READ_ONCE() as long as the pointer value is only used
  31  * for comparison and isn't being dereferenced.
  32  */
  33 static inline void rwsem_set_owner(struct rw_semaphore *sem)
  34 {
  35         WRITE_ONCE(sem->owner, current);
  36 }
  37
  38 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
  39 {
  40         WRITE_ONCE(sem->owner, NULL);
  41 }
  42
  43 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
  44 {
  45         /*
  46          * We check the owner value first to make sure that we will only
  47          * do a write to the rwsem cacheline when it is really necessary
  48          * to minimize cacheline contention.
  49          */
  50         if (READ_ONCE(sem->owner) != RWSEM_READER_OWNED)
  51                 WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
  52 }
  53
  54 static inline bool rwsem_owner_is_writer(struct task_struct *owner)
  55 {
  56         return owner && owner != RWSEM_READER_OWNED;
  57 }
  58
  59 static inline bool rwsem_owner_is_reader(struct task_struct *owner)
  60 {
  61         return owner == RWSEM_READER_OWNED;
  62 }
  63 #else
  64 static inline void rwsem_set_owner(struct rw_semaphore *sem)
  65 {
  66 }
  67
  68 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
  69 {
  70 }
  71
  72 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
  73 {
  74 }
  75 #endif