1 // Copyright 2017 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 #include <stdint.h>
16 #include <new>
17
18 // This file is a no-op if the required LowLevelAlloc support is missing.
19 #include "absl/base/internal/low_level_alloc.h"
20 #ifndef ABSL_LOW_LEVEL_ALLOC_MISSING
21
22 #include <string.h>
23
24 #include "absl/base/attributes.h"
25 #include "absl/base/internal/spinlock.h"
26 #include "absl/base/internal/thread_identity.h"
27 #include "absl/synchronization/internal/per_thread_sem.h"
28
29 namespace absl {
30 ABSL_NAMESPACE_BEGIN
31 namespace synchronization_internal {
32
33 // ThreadIdentity storage is persistent, we maintain a free-list of previously
34 // released ThreadIdentity objects.
35 static base_internal::SpinLock freelist_lock(
36 base_internal::kLinkerInitialized);
37 static base_internal::ThreadIdentity* thread_identity_freelist;
38
39 // A per-thread destructor for reclaiming associated ThreadIdentity objects.
40 // Since we must preserve their storage we cache them for re-use.
ReclaimThreadIdentity(void * v)41 void ReclaimThreadIdentity(void* v) {
42 base_internal::ThreadIdentity* identity =
43 static_cast<base_internal::ThreadIdentity*>(v);
44
45 // all_locks might have been allocated by the Mutex implementation.
46 // We free it here when we are notified that our thread is dying.
47 if (identity->per_thread_synch.all_locks != nullptr) {
48 base_internal::LowLevelAlloc::Free(identity->per_thread_synch.all_locks);
49 }
50
51 PerThreadSem::Destroy(identity);
52
53 // We must explicitly clear the current thread's identity:
54 // (a) Subsequent (unrelated) per-thread destructors may require an identity.
55 // We must guarantee a new identity is used in this case (this instructor
56 // will be reinvoked up to PTHREAD_DESTRUCTOR_ITERATIONS in this case).
57 // (b) ThreadIdentity implementations may depend on memory that is not
58 // reinitialized before reuse. We must allow explicit clearing of the
59 // association state in this case.
60 base_internal::ClearCurrentThreadIdentity();
61 {
62 base_internal::SpinLockHolder l(&freelist_lock);
63 identity->next = thread_identity_freelist;
64 thread_identity_freelist = identity;
65 }
66 }
67
68 // Return value rounded up to next multiple of align.
69 // Align must be a power of two.
RoundUp(intptr_t addr,intptr_t align)70 static intptr_t RoundUp(intptr_t addr, intptr_t align) {
71 return (addr + align - 1) & ~(align - 1);
72 }
73
ResetThreadIdentity(base_internal::ThreadIdentity * identity)74 static void ResetThreadIdentity(base_internal::ThreadIdentity* identity) {
75 base_internal::PerThreadSynch* pts = &identity->per_thread_synch;
76 pts->next = nullptr;
77 pts->skip = nullptr;
78 pts->may_skip = false;
79 pts->waitp = nullptr;
80 pts->suppress_fatal_errors = false;
81 pts->readers = 0;
82 pts->priority = 0;
83 pts->next_priority_read_cycles = 0;
84 pts->state.store(base_internal::PerThreadSynch::State::kAvailable,
85 std::memory_order_relaxed);
86 pts->maybe_unlocking = false;
87 pts->wake = false;
88 pts->cond_waiter = false;
89 pts->all_locks = nullptr;
90 identity->blocked_count_ptr = nullptr;
91 identity->ticker.store(0, std::memory_order_relaxed);
92 identity->wait_start.store(0, std::memory_order_relaxed);
93 identity->is_idle.store(false, std::memory_order_relaxed);
94 identity->next = nullptr;
95 }
96
NewThreadIdentity()97 static base_internal::ThreadIdentity* NewThreadIdentity() {
98 base_internal::ThreadIdentity* identity = nullptr;
99
100 {
101 // Re-use a previously released object if possible.
102 base_internal::SpinLockHolder l(&freelist_lock);
103 if (thread_identity_freelist) {
104 identity = thread_identity_freelist; // Take list-head.
105 thread_identity_freelist = thread_identity_freelist->next;
106 }
107 }
108
109 if (identity == nullptr) {
110 // Allocate enough space to align ThreadIdentity to a multiple of
111 // PerThreadSynch::kAlignment. This space is never released (it is
112 // added to a freelist by ReclaimThreadIdentity instead).
113 void* allocation = base_internal::LowLevelAlloc::Alloc(
114 sizeof(*identity) + base_internal::PerThreadSynch::kAlignment - 1);
115 // Round up the address to the required alignment.
116 identity = reinterpret_cast<base_internal::ThreadIdentity*>(
117 RoundUp(reinterpret_cast<intptr_t>(allocation),
118 base_internal::PerThreadSynch::kAlignment));
119 }
120 ResetThreadIdentity(identity);
121
122 return identity;
123 }
124
125 // Allocates and attaches ThreadIdentity object for the calling thread. Returns
126 // the new identity.
127 // REQUIRES: CurrentThreadIdentity(false) == nullptr
CreateThreadIdentity()128 base_internal::ThreadIdentity* CreateThreadIdentity() {
129 base_internal::ThreadIdentity* identity = NewThreadIdentity();
130 PerThreadSem::Init(identity);
131 // Associate the value with the current thread, and attach our destructor.
132 base_internal::SetCurrentThreadIdentity(identity, ReclaimThreadIdentity);
133 return identity;
134 }
135
136 } // namespace synchronization_internal
137 ABSL_NAMESPACE_END
138 } // namespace absl
139
140 #endif // ABSL_LOW_LEVEL_ALLOC_MISSING
141