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()/PERCPU_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 #include <linux/gfp.h>
  53
  54 struct percpu_ref;
  55 typedef void (percpu_ref_func_t)(struct percpu_ref *);
  56
  57 struct percpu_ref {
  58         atomic_long_t           count;
  59         /*
  60          * The low bit of the pointer indicates whether the ref is in percpu
  61          * mode; if set, then get/put will manipulate the atomic_t.
  62          */
  63         unsigned long           percpu_count_ptr;
  64         percpu_ref_func_t       *release;
  65         percpu_ref_func_t       *confirm_kill;
  66         struct rcu_head         rcu;
  67 };
  68
  69 int __must_check percpu_ref_init(struct percpu_ref *ref,
  70                                  percpu_ref_func_t *release, gfp_t gfp);
  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_reinit(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 __PERCPU_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->percpu_count is not NULL.
  98  */
  99 static inline bool __percpu_ref_alive(struct percpu_ref *ref,
 100                                       unsigned long __percpu **percpu_countp)
 101 {
 102         unsigned long percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr);
 103
 104         /* paired with smp_store_release() in percpu_ref_reinit() */
 105         smp_read_barrier_depends();
 106
 107         if (unlikely(percpu_ptr & __PERCPU_REF_DEAD))
 108                 return false;
 109
 110         *percpu_countp = (unsigned long __percpu *)percpu_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_long_inc().
 119  *
 120  * This function is safe to call as long as @ref is between init and exit.
 121  */
 122 static inline void percpu_ref_get(struct percpu_ref *ref)
 123 {
 124         unsigned long __percpu *percpu_count;
 125
 126         rcu_read_lock_sched();
 127
 128         if (__percpu_ref_alive(ref, &percpu_count))
 129                 this_cpu_inc(*percpu_count);
 130         else
 131                 atomic_long_inc(&ref->count);
 132
 133         rcu_read_unlock_sched();
 134 }
 135
 136 /**
 137  * percpu_ref_tryget - try to increment a percpu refcount
 138  * @ref: percpu_ref to try-get
 139  *
 140  * Increment a percpu refcount unless its count already reached zero.
 141  * Returns %true on success; %false on failure.
 142  *
 143  * This function is safe to call as long as @ref is between init and exit.
 144  */
 145 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 146 {
 147         unsigned long __percpu *percpu_count;
 148         int ret;
 149
 150         rcu_read_lock_sched();
 151
 152         if (__percpu_ref_alive(ref, &percpu_count)) {
 153                 this_cpu_inc(*percpu_count);
 154                 ret = true;
 155         } else {
 156                 ret = atomic_long_inc_not_zero(&ref->count);
 157         }
 158
 159         rcu_read_unlock_sched();
 160
 161         return ret;
 162 }
 163
 164 /**
 165  * percpu_ref_tryget_live - try to increment a live percpu refcount
 166  * @ref: percpu_ref to try-get
 167  *
 168  * Increment a percpu refcount unless it has already been killed.  Returns
 169  * %true on success; %false on failure.
 170  *
 171  * Completion of percpu_ref_kill() in itself doesn't guarantee that this
 172  * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
 173  * should be used.  After the confirm_kill callback is invoked, it's
 174  * guaranteed that no new reference will be given out by
 175  * percpu_ref_tryget_live().
 176  *
 177  * This function is safe to call as long as @ref is between init and exit.
 178  */
 179 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 180 {
 181         unsigned long __percpu *percpu_count;
 182         int ret = false;
 183
 184         rcu_read_lock_sched();
 185
 186         if (__percpu_ref_alive(ref, &percpu_count)) {
 187                 this_cpu_inc(*percpu_count);
 188                 ret = true;
 189         }
 190
 191         rcu_read_unlock_sched();
 192
 193         return ret;
 194 }
 195
 196 /**
 197  * percpu_ref_put - decrement a percpu refcount
 198  * @ref: percpu_ref to put
 199  *
 200  * Decrement the refcount, and if 0, call the release function (which was passed
 201  * to percpu_ref_init())
 202  *
 203  * This function is safe to call as long as @ref is between init and exit.
 204  */
 205 static inline void percpu_ref_put(struct percpu_ref *ref)
 206 {
 207         unsigned long __percpu *percpu_count;
 208
 209         rcu_read_lock_sched();
 210
 211         if (__percpu_ref_alive(ref, &percpu_count))
 212                 this_cpu_dec(*percpu_count);
 213         else if (unlikely(atomic_long_dec_and_test(&ref->count)))
 214                 ref->release(ref);
 215
 216         rcu_read_unlock_sched();
 217 }
 218
 219 /**
 220  * percpu_ref_is_zero - test whether a percpu refcount reached zero
 221  * @ref: percpu_ref to test
 222  *
 223  * Returns %true if @ref reached zero.
 224  *
 225  * This function is safe to call as long as @ref is between init and exit.
 226  */
 227 static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
 228 {
 229         unsigned long __percpu *percpu_count;
 230
 231         if (__percpu_ref_alive(ref, &percpu_count))
 232                 return false;
 233         return !atomic_long_read(&ref->count);
 234 }
 235
 236 #endif