1 #define pr_fmt(fmt) "%s: " fmt "\n", __func__
3 #include <linux/kernel.h>
4 #include <linux/percpu-refcount.h>
7 * Initially, a percpu refcount is just a set of percpu counters. Initially, we
8 * don't try to detect the ref hitting 0 - which means that get/put can just
9 * increment or decrement the local counter. Note that the counter on a
10 * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
11 * percpu counters will all sum to the correct value
13 * (More precisely: because moduler arithmatic is commutative the sum of all the
14 * pcpu_count vars will be equal to what it would have been if all the gets and
15 * puts were done to a single integer, even if some of the percpu integers
16 * overflow or underflow).
18 * The real trick to implementing percpu refcounts is shutdown. We can't detect
19 * the ref hitting 0 on every put - this would require global synchronization
20 * and defeat the whole purpose of using percpu refs.
22 * What we do is require the user to keep track of the initial refcount; we know
23 * the ref can't hit 0 before the user drops the initial ref, so as long as we
24 * convert to non percpu mode before the initial ref is dropped everything
27 * Converting to non percpu mode is done with some RCUish stuff in
28 * percpu_ref_kill. Additionally, we need a bias value so that the atomic_t
29 * can't hit 0 before we've added up all the percpu refs.
32 #define PCPU_COUNT_BIAS (1U << 31)
35 * percpu_ref_init - initialize a percpu refcount
36 * @ref: ref to initialize
37 * @release: function which will be called when refcount hits 0
39 * Initializes the refcount in single atomic counter mode with a refcount of 1;
40 * analagous to atomic_set(ref, 1).
42 * Note that @release must not sleep - it may potentially be called from RCU
43 * callback context by percpu_ref_kill().
45 int percpu_ref_init(struct percpu_ref
*ref
, percpu_ref_release
*release
)
47 atomic_set(&ref
->count
, 1 + PCPU_COUNT_BIAS
);
49 ref
->pcpu_count
= alloc_percpu(unsigned);
53 ref
->release
= release
;
57 static void percpu_ref_kill_rcu(struct rcu_head
*rcu
)
59 struct percpu_ref
*ref
= container_of(rcu
, struct percpu_ref
, rcu
);
60 unsigned __percpu
*pcpu_count
;
64 pcpu_count
= ACCESS_ONCE(ref
->pcpu_count
);
66 /* Mask out PCPU_REF_DEAD */
67 pcpu_count
= (unsigned __percpu
*)
68 (((unsigned long) pcpu_count
) & ~PCPU_STATUS_MASK
);
70 for_each_possible_cpu(cpu
)
71 count
+= *per_cpu_ptr(pcpu_count
, cpu
);
73 free_percpu(pcpu_count
);
75 pr_debug("global %i pcpu %i", atomic_read(&ref
->count
), (int) count
);
78 * It's crucial that we sum the percpu counters _before_ adding the sum
79 * to &ref->count; since gets could be happening on one cpu while puts
80 * happen on another, adding a single cpu's count could cause
81 * @ref->count to hit 0 before we've got a consistent value - but the
82 * sum of all the counts will be consistent and correct.
84 * Subtracting the bias value then has to happen _after_ adding count to
85 * &ref->count; we need the bias value to prevent &ref->count from
86 * reaching 0 before we add the percpu counts. But doing it at the same
87 * time is equivalent and saves us atomic operations:
90 atomic_add((int) count
- PCPU_COUNT_BIAS
, &ref
->count
);
93 * Now we're in single atomic_t mode with a consistent refcount, so it's
94 * safe to drop our initial ref:
100 * percpu_ref_kill - safely drop initial ref
102 * Must be used to drop the initial ref on a percpu refcount; must be called
103 * precisely once before shutdown.
105 * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
106 * percpu counters and dropping the initial ref.
108 void percpu_ref_kill(struct percpu_ref
*ref
)
110 WARN_ONCE(REF_STATUS(ref
->pcpu_count
) == PCPU_REF_DEAD
,
111 "percpu_ref_kill() called more than once!\n");
113 ref
->pcpu_count
= (unsigned __percpu
*)
114 (((unsigned long) ref
->pcpu_count
)|PCPU_REF_DEAD
);
116 call_rcu(&ref
->rcu
, percpu_ref_kill_rcu
);
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