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 /* flags set in the lower bits of percpu_ref->percpu_count_ptr */
  58 enum {
  59         __PERCPU_REF_ATOMIC     = 1LU << 0,     /* operating in atomic mode */
  60 };
  61
  62 struct percpu_ref {
  63         atomic_long_t           count;
  64         /*
  65          * The low bit of the pointer indicates whether the ref is in percpu
  66          * mode; if set, then get/put will manipulate the atomic_t.
  67          */
  68         unsigned long           percpu_count_ptr;
  69         percpu_ref_func_t       *release;
  70         percpu_ref_func_t       *confirm_switch;
  71         struct rcu_head         rcu;
  72 };
  73
  74 int __must_check percpu_ref_init(struct percpu_ref *ref,
  75                                  percpu_ref_func_t *release, gfp_t gfp);
  76 void percpu_ref_exit(struct percpu_ref *ref);
  77 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
  78                                  percpu_ref_func_t *confirm_kill);
  79 void percpu_ref_reinit(struct percpu_ref *ref);
  80
  81 /**
  82  * percpu_ref_kill - drop the initial ref
  83  * @ref: percpu_ref to kill
  84  *
  85  * Must be used to drop the initial ref on a percpu refcount; must be called
  86  * precisely once before shutdown.
  87  *
  88  * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
  89  * percpu counters and dropping the initial ref.
  90  */
  91 static inline void percpu_ref_kill(struct percpu_ref *ref)
  92 {
  93         return percpu_ref_kill_and_confirm(ref, NULL);
  94 }
  95
  96 /*
  97  * Internal helper.  Don't use outside percpu-refcount proper.  The
  98  * function doesn't return the pointer and let the caller test it for NULL
  99  * because doing so forces the compiler to generate two conditional
 100  * branches as it can't assume that @ref->percpu_count is not NULL.
 101  */
 102 static inline bool __ref_is_percpu(struct percpu_ref *ref,
 103                                           unsigned long __percpu **percpu_countp)
 104 {
 105         unsigned long percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr);
 106
 107         /* paired with smp_store_release() in percpu_ref_reinit() */
 108         smp_read_barrier_depends();
 109
 110         if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC))
 111                 return false;
 112
 113         *percpu_countp = (unsigned long __percpu *)percpu_ptr;
 114         return true;
 115 }
 116
 117 /**
 118  * percpu_ref_get - increment a percpu refcount
 119  * @ref: percpu_ref to get
 120  *
 121  * Analagous to atomic_long_inc().
 122  *
 123  * This function is safe to call as long as @ref is between init and exit.
 124  */
 125 static inline void percpu_ref_get(struct percpu_ref *ref)
 126 {
 127         unsigned long __percpu *percpu_count;
 128
 129         rcu_read_lock_sched();
 130
 131         if (__ref_is_percpu(ref, &percpu_count))
 132                 this_cpu_inc(*percpu_count);
 133         else
 134                 atomic_long_inc(&ref->count);
 135
 136         rcu_read_unlock_sched();
 137 }
 138
 139 /**
 140  * percpu_ref_tryget - try to increment a percpu refcount
 141  * @ref: percpu_ref to try-get
 142  *
 143  * Increment a percpu refcount unless its count already reached zero.
 144  * Returns %true on success; %false on failure.
 145  *
 146  * This function is safe to call as long as @ref is between init and exit.
 147  */
 148 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 149 {
 150         unsigned long __percpu *percpu_count;
 151         int ret;
 152
 153         rcu_read_lock_sched();
 154
 155         if (__ref_is_percpu(ref, &percpu_count)) {
 156                 this_cpu_inc(*percpu_count);
 157                 ret = true;
 158         } else {
 159                 ret = atomic_long_inc_not_zero(&ref->count);
 160         }
 161
 162         rcu_read_unlock_sched();
 163
 164         return ret;
 165 }
 166
 167 /**
 168  * percpu_ref_tryget_live - try to increment a live percpu refcount
 169  * @ref: percpu_ref to try-get
 170  *
 171  * Increment a percpu refcount unless it has already been killed.  Returns
 172  * %true on success; %false on failure.
 173  *
 174  * Completion of percpu_ref_kill() in itself doesn't guarantee that this
 175  * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
 176  * should be used.  After the confirm_kill callback is invoked, it's
 177  * guaranteed that no new reference will be given out by
 178  * percpu_ref_tryget_live().
 179  *
 180  * This function is safe to call as long as @ref is between init and exit.
 181  */
 182 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 183 {
 184         unsigned long __percpu *percpu_count;
 185         int ret = false;
 186
 187         rcu_read_lock_sched();
 188
 189         if (__ref_is_percpu(ref, &percpu_count)) {
 190                 this_cpu_inc(*percpu_count);
 191                 ret = true;
 192         }
 193
 194         rcu_read_unlock_sched();
 195
 196         return ret;
 197 }
 198
 199 /**
 200  * percpu_ref_put - decrement a percpu refcount
 201  * @ref: percpu_ref to put
 202  *
 203  * Decrement the refcount, and if 0, call the release function (which was passed
 204  * to percpu_ref_init())
 205  *
 206  * This function is safe to call as long as @ref is between init and exit.
 207  */
 208 static inline void percpu_ref_put(struct percpu_ref *ref)
 209 {
 210         unsigned long __percpu *percpu_count;
 211
 212         rcu_read_lock_sched();
 213
 214         if (__ref_is_percpu(ref, &percpu_count))
 215                 this_cpu_dec(*percpu_count);
 216         else if (unlikely(atomic_long_dec_and_test(&ref->count)))
 217                 ref->release(ref);
 218
 219         rcu_read_unlock_sched();
 220 }
 221
 222 /**
 223  * percpu_ref_is_zero - test whether a percpu refcount reached zero
 224  * @ref: percpu_ref to test
 225  *
 226  * Returns %true if @ref reached zero.
 227  *
 228  * This function is safe to call as long as @ref is between init and exit.
 229  */
 230 static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
 231 {
 232         unsigned long __percpu *percpu_count;
 233
 234         if (__ref_is_percpu(ref, &percpu_count))
 235                 return false;
 236         return !atomic_long_read(&ref->count);
 237 }
 238
 239 #endif