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 synchronize_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         __PERCPU_REF_DEAD       = 1LU << 1,     /* (being) killed */
  61         __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
  62
  63         __PERCPU_REF_FLAG_BITS  = 2,
  64 };
  65
  66 /* @flags for percpu_ref_init() */
  67 enum {
  68         /*
  69          * Start w/ ref == 1 in atomic mode.  Can be switched to percpu
  70          * operation using percpu_ref_switch_to_percpu().  If initialized
  71          * with this flag, the ref will stay in atomic mode until
  72          * percpu_ref_switch_to_percpu() is invoked on it.
  73          */
  74         PERCPU_REF_INIT_ATOMIC  = 1 << 0,
  75
  76         /*
  77          * Start dead w/ ref == 0 in atomic mode.  Must be revived with
  78          * percpu_ref_reinit() before used.  Implies INIT_ATOMIC.
  79          */
  80         PERCPU_REF_INIT_DEAD    = 1 << 1,
  81 };
  82
  83 struct percpu_ref {
  84         atomic_long_t           count;
  85         /*
  86          * The low bit of the pointer indicates whether the ref is in percpu
  87          * mode; if set, then get/put will manipulate the atomic_t.
  88          */
  89         unsigned long           percpu_count_ptr;
  90         percpu_ref_func_t       *release;
  91         percpu_ref_func_t       *confirm_switch;
  92         bool                    force_atomic:1;
  93         struct rcu_head         rcu;
  94 };
  95
  96 int __must_check percpu_ref_init(struct percpu_ref *ref,
  97                                  percpu_ref_func_t *release, unsigned int flags,
  98                                  gfp_t gfp);
  99 void percpu_ref_exit(struct percpu_ref *ref);
 100 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 101                                  percpu_ref_func_t *confirm_switch);
 102 void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
 103 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 104                                  percpu_ref_func_t *confirm_kill);
 105 void percpu_ref_reinit(struct percpu_ref *ref);
 106
 107 /**
 108  * percpu_ref_kill - drop the initial ref
 109  * @ref: percpu_ref to kill
 110  *
 111  * Must be used to drop the initial ref on a percpu refcount; must be called
 112  * precisely once before shutdown.
 113  *
 114  * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
 115  * percpu counters and dropping the initial ref.
 116  */
 117 static inline void percpu_ref_kill(struct percpu_ref *ref)
 118 {
 119         return percpu_ref_kill_and_confirm(ref, NULL);
 120 }
 121
 122 /*
 123  * Internal helper.  Don't use outside percpu-refcount proper.  The
 124  * function doesn't return the pointer and let the caller test it for NULL
 125  * because doing so forces the compiler to generate two conditional
 126  * branches as it can't assume that @ref->percpu_count is not NULL.
 127  */
 128 static inline bool __ref_is_percpu(struct percpu_ref *ref,
 129                                           unsigned long __percpu **percpu_countp)
 130 {
 131         unsigned long percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr);
 132
 133         /* paired with smp_store_release() in percpu_ref_reinit() */
 134         smp_read_barrier_depends();
 135
 136         /*
 137          * Theoretically, the following could test just ATOMIC; however,
 138          * then we'd have to mask off DEAD separately as DEAD may be
 139          * visible without ATOMIC if we race with percpu_ref_kill().  DEAD
 140          * implies ATOMIC anyway.  Test them together.
 141          */
 142         if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
 143                 return false;
 144
 145         *percpu_countp = (unsigned long __percpu *)percpu_ptr;
 146         return true;
 147 }
 148
 149 /**
 150  * percpu_ref_get - increment a percpu refcount
 151  * @ref: percpu_ref to get
 152  *
 153  * Analagous to atomic_long_inc().
 154  *
 155  * This function is safe to call as long as @ref is between init and exit.
 156  */
 157 static inline void percpu_ref_get(struct percpu_ref *ref)
 158 {
 159         unsigned long __percpu *percpu_count;
 160
 161         rcu_read_lock_sched();
 162
 163         if (__ref_is_percpu(ref, &percpu_count))
 164                 this_cpu_inc(*percpu_count);
 165         else
 166                 atomic_long_inc(&ref->count);
 167
 168         rcu_read_unlock_sched();
 169 }
 170
 171 /**
 172  * percpu_ref_tryget - try to increment a percpu refcount
 173  * @ref: percpu_ref to try-get
 174  *
 175  * Increment a percpu refcount unless its count already reached zero.
 176  * Returns %true on success; %false on failure.
 177  *
 178  * This function is safe to call as long as @ref is between init and exit.
 179  */
 180 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 181 {
 182         unsigned long __percpu *percpu_count;
 183         int ret;
 184
 185         rcu_read_lock_sched();
 186
 187         if (__ref_is_percpu(ref, &percpu_count)) {
 188                 this_cpu_inc(*percpu_count);
 189                 ret = true;
 190         } else {
 191                 ret = atomic_long_inc_not_zero(&ref->count);
 192         }
 193
 194         rcu_read_unlock_sched();
 195
 196         return ret;
 197 }
 198
 199 /**
 200  * percpu_ref_tryget_live - try to increment a live percpu refcount
 201  * @ref: percpu_ref to try-get
 202  *
 203  * Increment a percpu refcount unless it has already been killed.  Returns
 204  * %true on success; %false on failure.
 205  *
 206  * Completion of percpu_ref_kill() in itself doesn't guarantee that this
 207  * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
 208  * should be used.  After the confirm_kill callback is invoked, it's
 209  * guaranteed that no new reference will be given out by
 210  * percpu_ref_tryget_live().
 211  *
 212  * This function is safe to call as long as @ref is between init and exit.
 213  */
 214 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 215 {
 216         unsigned long __percpu *percpu_count;
 217         int ret = false;
 218
 219         rcu_read_lock_sched();
 220
 221         if (__ref_is_percpu(ref, &percpu_count)) {
 222                 this_cpu_inc(*percpu_count);
 223                 ret = true;
 224         } else if (!(ACCESS_ONCE(ref->percpu_count_ptr) & __PERCPU_REF_DEAD)) {
 225                 ret = atomic_long_inc_not_zero(&ref->count);
 226         }
 227
 228         rcu_read_unlock_sched();
 229
 230         return ret;
 231 }
 232
 233 /**
 234  * percpu_ref_put - decrement a percpu refcount
 235  * @ref: percpu_ref to put
 236  *
 237  * Decrement the refcount, and if 0, call the release function (which was passed
 238  * to percpu_ref_init())
 239  *
 240  * This function is safe to call as long as @ref is between init and exit.
 241  */
 242 static inline void percpu_ref_put(struct percpu_ref *ref)
 243 {
 244         unsigned long __percpu *percpu_count;
 245
 246         rcu_read_lock_sched();
 247
 248         if (__ref_is_percpu(ref, &percpu_count))
 249                 this_cpu_dec(*percpu_count);
 250         else if (unlikely(atomic_long_dec_and_test(&ref->count)))
 251                 ref->release(ref);
 252
 253         rcu_read_unlock_sched();
 254 }
 255
 256 /**
 257  * percpu_ref_is_zero - test whether a percpu refcount reached zero
 258  * @ref: percpu_ref to test
 259  *
 260  * Returns %true if @ref reached zero.
 261  *
 262  * This function is safe to call as long as @ref is between init and exit.
 263  */
 264 static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
 265 {
 266         unsigned long __percpu *percpu_count;
 267
 268         if (__ref_is_percpu(ref, &percpu_count))
 269                 return false;
 270         return !atomic_long_read(&ref->count);
 271 }
 272
 273 #endif