include/linux/percpu-refcount.h

   1 /*
   2  * Percpu refcounts:
   3  * (C) 2012 Google, Inc.
   4  * Author: Kent Overstreet <koverstreet@google.com>
   5  *
   6  * This implements a refcount with similar semantics to atomic_t - atomic_inc(),
   7  * atomic_dec_and_test() - but percpu.
   8  *
   9  * There's one important difference between percpu refs and normal atomic_t
  10  * refcounts; you have to keep track of your initial refcount, and then when you
  11  * start shutting down you call percpu_ref_kill() _before_ dropping the initial
  12  * refcount.
  13  *
  14  * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less
  15  * than an atomic_t - this is because of the way shutdown works, see
  16  * percpu_ref_kill()/PCPU_COUNT_BIAS.
  17  *
  18  * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the
  19  * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill()
  20  * puts the ref back in single atomic_t mode, collecting the per cpu refs and
  21  * issuing the appropriate barriers, and then marks the ref as shutting down so
  22  * that percpu_ref_put() will check for the ref hitting 0.  After it returns,
  23  * it's safe to drop the initial ref.
  24  *
  25  * USAGE:
  26  *
  27  * See fs/aio.c for some example usage; it's used there for struct kioctx, which
  28  * is created when userspaces calls io_setup(), and destroyed when userspace
  29  * calls io_destroy() or the process exits.
  30  *
  31  * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it
  32  * calls percpu_ref_kill(), then hlist_del_rcu() and sychronize_rcu() to remove
  33  * the kioctx from the proccess's list of kioctxs - after that, there can't be
  34  * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop
  35  * the initial ref with percpu_ref_put().
  36  *
  37  * Code that does a two stage shutdown like this often needs some kind of
  38  * explicit synchronization to ensure the initial refcount can only be dropped
  39  * once - percpu_ref_kill() does this for you, it returns true once and false if
  40  * someone else already called it. The aio code uses it this way, but it's not
  41  * necessary if the code has some other mechanism to synchronize teardown.
  42  * around.
  43  */
  44
  45 #ifndef _LINUX_PERCPU_REFCOUNT_H
  46 #define _LINUX_PERCPU_REFCOUNT_H
  47
  48 #include <linux/atomic.h>
  49 #include <linux/kernel.h>
  50 #include <linux/percpu.h>
  51 #include <linux/rcupdate.h>
  52
  53 struct percpu_ref;
  54 typedef void (percpu_ref_func_t)(struct percpu_ref *);
  55
  56 struct percpu_ref {
  57         atomic_t                count;
  58         /*
  59          * The low bit of the pointer indicates whether the ref is in percpu
  60          * mode; if set, then get/put will manipulate the atomic_t (this is a
  61          * hack because we need to keep the pointer around for
  62          * percpu_ref_kill_rcu())
  63          */
  64         unsigned __percpu       *pcpu_count;
  65         percpu_ref_func_t       *release;
  66         percpu_ref_func_t       *confirm_kill;
  67         struct rcu_head         rcu;
  68 };
  69
  70 int __must_check percpu_ref_init(struct percpu_ref *ref,
  71                                  percpu_ref_func_t *release);
  72 void percpu_ref_cancel_init(struct percpu_ref *ref);
  73 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
  74                                  percpu_ref_func_t *confirm_kill);
  75
  76 /**
  77  * percpu_ref_kill - drop the initial ref
  78  * @ref: percpu_ref to kill
  79  *
  80  * Must be used to drop the initial ref on a percpu refcount; must be called
  81  * precisely once before shutdown.
  82  *
  83  * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
  84  * percpu counters and dropping the initial ref.
  85  */
  86 static inline void percpu_ref_kill(struct percpu_ref *ref)
  87 {
  88         return percpu_ref_kill_and_confirm(ref, NULL);
  89 }
  90
  91 #define PCPU_STATUS_BITS        2
  92 #define PCPU_STATUS_MASK        ((1 << PCPU_STATUS_BITS) - 1)
  93 #define PCPU_REF_PTR            0
  94 #define PCPU_REF_DEAD           1
  95
  96 #define REF_STATUS(count)       (((unsigned long) count) & PCPU_STATUS_MASK)
  97
  98 /**
  99  * percpu_ref_get - increment a percpu refcount
 100  * @ref: percpu_ref to get
 101  *
 102  * Analagous to atomic_inc().
 103   */
 104 static inline void percpu_ref_get(struct percpu_ref *ref)
 105 {
 106         unsigned __percpu *pcpu_count;
 107
 108         rcu_read_lock_sched();
 109
 110         pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 111
 112         if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
 113                 __this_cpu_inc(*pcpu_count);
 114         else
 115                 atomic_inc(&ref->count);
 116
 117         rcu_read_unlock_sched();
 118 }
 119
 120 /**
 121  * percpu_ref_tryget - try to increment a percpu refcount
 122  * @ref: percpu_ref to try-get
 123  *
 124  * Increment a percpu refcount unless it has already been killed.  Returns
 125  * %true on success; %false on failure.
 126  *
 127  * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget
 128  * will fail.  For such guarantee, percpu_ref_kill_and_confirm() should be
 129  * used.  After the confirm_kill callback is invoked, it's guaranteed that
 130  * no new reference will be given out by percpu_ref_tryget().
 131  */
 132 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 133 {
 134         unsigned __percpu *pcpu_count;
 135         int ret = false;
 136
 137         rcu_read_lock_sched();
 138
 139         pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 140
 141         if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) {
 142                 __this_cpu_inc(*pcpu_count);
 143                 ret = true;
 144         }
 145
 146         rcu_read_unlock_sched();
 147
 148         return ret;
 149 }
 150
 151 /**
 152  * percpu_ref_put - decrement a percpu refcount
 153  * @ref: percpu_ref to put
 154  *
 155  * Decrement the refcount, and if 0, call the release function (which was passed
 156  * to percpu_ref_init())
 157  */
 158 static inline void percpu_ref_put(struct percpu_ref *ref)
 159 {
 160         unsigned __percpu *pcpu_count;
 161
 162         rcu_read_lock_sched();
 163
 164         pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 165
 166         if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
 167                 __this_cpu_dec(*pcpu_count);
 168         else if (unlikely(atomic_dec_and_test(&ref->count)))
 169                 ref->release(ref);
 170
 171         rcu_read_unlock_sched();
 172 }
 173
 174 #endif