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.
  61          */
  62         unsigned long           pcpu_count_ptr;
  63         percpu_ref_func_t       *release;
  64         percpu_ref_func_t       *confirm_kill;
  65         struct rcu_head         rcu;
  66 };
  67
  68 int __must_check percpu_ref_init(struct percpu_ref *ref,
  69                                  percpu_ref_func_t *release);
  70 void percpu_ref_reinit(struct percpu_ref *ref);
  71 void percpu_ref_exit(struct percpu_ref *ref);
  72 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
  73                                  percpu_ref_func_t *confirm_kill);
  74 void __percpu_ref_kill_expedited(struct percpu_ref *ref);
  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_REF_DEAD           1
  92
  93 /*
  94  * Internal helper.  Don't use outside percpu-refcount proper.  The
  95  * function doesn't return the pointer and let the caller test it for NULL
  96  * because doing so forces the compiler to generate two conditional
  97  * branches as it can't assume that @ref->pcpu_count is not NULL.
  98  */
  99 static inline bool __pcpu_ref_alive(struct percpu_ref *ref,
 100                                     unsigned __percpu **pcpu_countp)
 101 {
 102         unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr);
 103
 104         /* paired with smp_store_release() in percpu_ref_reinit() */
 105         smp_read_barrier_depends();
 106
 107         if (unlikely(pcpu_ptr & PCPU_REF_DEAD))
 108                 return false;
 109
 110         *pcpu_countp = (unsigned __percpu *)pcpu_ptr;
 111         return true;
 112 }
 113
 114 /**
 115  * percpu_ref_get - increment a percpu refcount
 116  * @ref: percpu_ref to get
 117  *
 118  * Analagous to atomic_inc().
 119   */
 120 static inline void percpu_ref_get(struct percpu_ref *ref)
 121 {
 122         unsigned __percpu *pcpu_count;
 123
 124         rcu_read_lock_sched();
 125
 126         if (__pcpu_ref_alive(ref, &pcpu_count))
 127                 this_cpu_inc(*pcpu_count);
 128         else
 129                 atomic_inc(&ref->count);
 130
 131         rcu_read_unlock_sched();
 132 }
 133
 134 /**
 135  * percpu_ref_tryget - try to increment a percpu refcount
 136  * @ref: percpu_ref to try-get
 137  *
 138  * Increment a percpu refcount unless its count already reached zero.
 139  * Returns %true on success; %false on failure.
 140  *
 141  * The caller is responsible for ensuring that @ref stays accessible.
 142  */
 143 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 144 {
 145         unsigned __percpu *pcpu_count;
 146         int ret = false;
 147
 148         rcu_read_lock_sched();
 149
 150         if (__pcpu_ref_alive(ref, &pcpu_count)) {
 151                 this_cpu_inc(*pcpu_count);
 152                 ret = true;
 153         } else {
 154                 ret = atomic_inc_not_zero(&ref->count);
 155         }
 156
 157         rcu_read_unlock_sched();
 158
 159         return ret;
 160 }
 161
 162 /**
 163  * percpu_ref_tryget_live - try to increment a live percpu refcount
 164  * @ref: percpu_ref to try-get
 165  *
 166  * Increment a percpu refcount unless it has already been killed.  Returns
 167  * %true on success; %false on failure.
 168  *
 169  * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget
 170  * will fail.  For such guarantee, percpu_ref_kill_and_confirm() should be
 171  * used.  After the confirm_kill callback is invoked, it's guaranteed that
 172  * no new reference will be given out by percpu_ref_tryget().
 173  *
 174  * The caller is responsible for ensuring that @ref stays accessible.
 175  */
 176 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 177 {
 178         unsigned __percpu *pcpu_count;
 179         int ret = false;
 180
 181         rcu_read_lock_sched();
 182
 183         if (__pcpu_ref_alive(ref, &pcpu_count)) {
 184                 this_cpu_inc(*pcpu_count);
 185                 ret = true;
 186         }
 187
 188         rcu_read_unlock_sched();
 189
 190         return ret;
 191 }
 192
 193 /**
 194  * percpu_ref_put - decrement a percpu refcount
 195  * @ref: percpu_ref to put
 196  *
 197  * Decrement the refcount, and if 0, call the release function (which was passed
 198  * to percpu_ref_init())
 199  */
 200 static inline void percpu_ref_put(struct percpu_ref *ref)
 201 {
 202         unsigned __percpu *pcpu_count;
 203
 204         rcu_read_lock_sched();
 205
 206         if (__pcpu_ref_alive(ref, &pcpu_count))
 207                 this_cpu_dec(*pcpu_count);
 208         else if (unlikely(atomic_dec_and_test(&ref->count)))
 209                 ref->release(ref);
 210
 211         rcu_read_unlock_sched();
 212 }
 213
 214 /**
 215  * percpu_ref_is_zero - test whether a percpu refcount reached zero
 216  * @ref: percpu_ref to test
 217  *
 218  * Returns %true if @ref reached zero.
 219  */
 220 static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
 221 {
 222         unsigned __percpu *pcpu_count;
 223
 224         if (__pcpu_ref_alive(ref, &pcpu_count))
 225                 return false;
 226         return !atomic_read(&ref->count);
 227 }
 228
 229 #endif