1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
|
#include "access.h"
#include "variable.h"
#include <library/cpp/yt/memory/leaky_singleton.h>
#include <array>
namespace NYT::NGlobal {
////////////////////////////////////////////////////////////////////////////////
namespace NDetail {
inline constexpr int MaxTrackedGlobalVariables = 32;
////////////////////////////////////////////////////////////////////////////////
class TGlobalVariablesRegistry
{
public:
static TGlobalVariablesRegistry* Get()
{
return LeakySingleton<TGlobalVariablesRegistry>();
}
void RegisterAccessor(const TVariableTag& tag, TAccessor accessor)
{
if (!tag.Initialized_.exchange(true, std::memory_order::relaxed)) { // (a)
DoRegisterAccessor(tag, accessor);
return;
}
TryVerifyingExistingAccessor(tag, accessor);
}
std::optional<TErasedStorage> GetVariable(const TVariableTag& tag)
{
auto key = tag.Key_.load(std::memory_order::acquire); // (e)
if (key != -1) {
return Accessors_[key](); // (f)
}
return std::nullopt;
}
private:
std::atomic<int> KeyGenerator_ = 0;
std::array<TAccessor, MaxTrackedGlobalVariables> Accessors_;
void DoRegisterAccessor(const TVariableTag& tag, TAccessor accessor)
{
// Get id -> place accessor -> store id
auto key = KeyGenerator_.fetch_add(1, std::memory_order::relaxed); // (b)
YT_VERIFY(key < MaxTrackedGlobalVariables);
Accessors_[key] = accessor; // (c)
tag.Key_.store(key, std::memory_order::release); // (d)
}
void TryVerifyingExistingAccessor(const TVariableTag& tag, TAccessor accessor)
{
auto key = tag.Key_.load(std::memory_order::acquire); // (e')
if (key == -1) {
// Accessor is about to be set.
// In order to avoid deadlock caused by forks
// we just leave. We could try acquiring fork
// locks here but this makes our check too expensive
// to be bothered.
return;
}
// Accessor has been already set -> safe to read it.
YT_VERIFY(Accessors_[key] == accessor); // (f')
}
};
// (arkady-e1ppa): Memory orders:
/*
We have two scenarios: 2 writes and write & read:
2 writes: Accessors_ is protected via Initialized_ flag
and KeyGenerator_ counter.
1) RMW (a) reads the last value in modification order
thus relaxed is enough to ensure <= 1 threads registering
per Tag.
2) KeyGenerator_ uses the same logic (see (b))
to ensure <= 1 threads registering per index in array.
If there are two writes per tag, then there is a "losing"
thread which read Initialized_ // true. For all intents
and purposes TryVerifyingExistingAccessor call is identical
to GetVariable call.
write & read: Relevant execution is below
W^na(Accessors_[id], 0x0) // Ctor
T1(Register) T2(Read)
W^na(Accessors_[id], 0x42) (c) R^acq(Key_, id) (e)
W^rel(Key_, id) (d) R^na(Accessors_[id], 0x42) (f)
(d) -rf-> (e) => (d) -SW-> (e). Since (c) -SB-> (d) and (e) -SB-> (f)
we have (c) -strongly HB-> (f) (strongly happens before). Thus we must
read 0x42 from Accessors_[id] (and not 0x0 which was written in ctor).
*/
////////////////////////////////////////////////////////////////////////////////
void RegisterVariable(const TVariableTag& tag, TAccessor accessor)
{
TGlobalVariablesRegistry::Get()->RegisterAccessor(tag, accessor);
}
} // namespace NDetail
////////////////////////////////////////////////////////////////////////////////
std::optional<TErasedStorage> GetErasedVariable(const TVariableTag& tag)
{
return NDetail::TGlobalVariablesRegistry::Get()->GetVariable(tag);
}
////////////////////////////////////////////////////////////////////////////////
} // namespace NYT::NGlobal
|
