| Directory: | cvmfs/ |
|---|---|
| File: | cvmfs/fence.h |
| Date: | 2026-10-11 02:40:02 |
| Exec | Total | Coverage | |
|---|---|---|---|
| Lines: | 23 | 25 | 92.0% |
| Branches: | 2 | 4 | 50.0% |
| Line | Branch | Exec | Source |
|---|---|---|---|
| 1 | /** | ||
| 2 | * This file is part of the CernVM File System. | ||
| 3 | */ | ||
| 4 | |||
| 5 | #ifndef CVMFS_FENCE_H_ | ||
| 6 | #define CVMFS_FENCE_H_ | ||
| 7 | |||
| 8 | #include <atomic> | ||
| 9 | |||
| 10 | #include "duplex_testing.h" | ||
| 11 | #include "util/posix.h" | ||
| 12 | #include "util/single_copy.h" | ||
| 13 | |||
| 14 | #ifdef CVMFS_NAMESPACE_GUARD | ||
| 15 | namespace CVMFS_NAMESPACE_GUARD { | ||
| 16 | #endif | ||
| 17 | |||
| 18 | /** | ||
| 19 | * A Fence can be used to protect critical regions where blocking is a very | ||
| 20 | * rare operation. When the Fence is not blocked, entering and leaving a | ||
| 21 | * critical region requires only a 1-2 atomic operations. In order to block | ||
| 22 | * the fence, no new threads can enter a critical region. When all entered | ||
| 23 | * regions are left, the fence is blocked. Waiting is done through slow busy | ||
| 24 | * wait. | ||
| 25 | */ | ||
| 26 | class Fence : public SingleCopy { | ||
| 27 | FRIEND_TEST(T_Fence, Basics); | ||
| 28 | |||
| 29 | public: | ||
| 30 | 43 | Fence() { | |
| 31 | 43 | counter_.store(0); | |
| 32 | 43 | blocking_.store(0); | |
| 33 | 43 | } | |
| 34 | |||
| 35 | 86 | void Enter() { | |
| 36 |
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172 | while (blocking_.load()) { |
| 37 | ✗ | SafeSleepMs(kBusyWaitBackoffMs); | |
| 38 | } | ||
| 39 | 86 | counter_.fetch_add(1); | |
| 40 | 86 | } | |
| 41 | |||
| 42 | 86 | void Leave() { counter_.fetch_sub(1); } | |
| 43 | |||
| 44 | 86 | void Close() { | |
| 45 | 86 | int32_t expected_val = 0; | |
| 46 | 86 | blocking_.compare_exchange_strong(expected_val, 1); | |
| 47 | 86 | } | |
| 48 | |||
| 49 | /** | ||
| 50 | * Close and let live critical regions exit | ||
| 51 | */ | ||
| 52 | 43 | void Drain() { | |
| 53 | 43 | Close(); | |
| 54 |
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86 | while (counter_.load() > 0) { |
| 55 | ✗ | SafeSleepMs(kBusyWaitBackoffMs); | |
| 56 | } | ||
| 57 | 43 | } | |
| 58 | |||
| 59 | 86 | void Open() { | |
| 60 | 86 | int32_t expected_val = 1; | |
| 61 | 86 | blocking_.compare_exchange_strong(expected_val, 0); | |
| 62 | 86 | } | |
| 63 | |||
| 64 | private: | ||
| 65 | static const unsigned kBusyWaitBackoffMs = 100; | ||
| 66 | |||
| 67 | /** | ||
| 68 | * Number of active critical regions. | ||
| 69 | */ | ||
| 70 | std::atomic<int64_t> counter_; | ||
| 71 | |||
| 72 | /** | ||
| 73 | * A boolean that indicates if the fence is blocked. | ||
| 74 | */ | ||
| 75 | std::atomic<int32_t> blocking_; | ||
| 76 | }; | ||
| 77 | |||
| 78 | |||
| 79 | /** | ||
| 80 | * RAII wrapper in case an entire function or code block should be protected | ||
| 81 | * by a fence. | ||
| 82 | */ | ||
| 83 | class FenceGuard { | ||
| 84 | public: | ||
| 85 | 43 | explicit FenceGuard(Fence *fence) : fence_(fence) { fence_->Enter(); } | |
| 86 | 43 | ~FenceGuard() { fence_->Leave(); } | |
| 87 | |||
| 88 | private: | ||
| 89 | Fence *fence_; | ||
| 90 | }; | ||
| 91 | |||
| 92 | #ifdef CVMFS_NAMESPACE_GUARD | ||
| 93 | } // namespace CVMFS_NAMESPACE_GUARD | ||
| 94 | #endif | ||
| 95 | |||
| 96 | #endif // CVMFS_FENCE_H_ | ||
| 97 |