| Directory: | cvmfs/ |
|---|---|
| File: | cvmfs/fence.h |
| Date: | 2025-06-22 02:36:02 |
| Exec | Total | Coverage | |
|---|---|---|---|
| Lines: | 17 | 19 | 89.5% |
| 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 "gtest/gtest_prod.h" | ||
| 9 | #include "util/atomic.h" | ||
| 10 | #include "util/posix.h" | ||
| 11 | #include "util/single_copy.h" | ||
| 12 | |||
| 13 | #ifdef CVMFS_NAMESPACE_GUARD | ||
| 14 | namespace CVMFS_NAMESPACE_GUARD { | ||
| 15 | #endif | ||
| 16 | |||
| 17 | /** | ||
| 18 | * A Fence can be used to protect critical regions where blocking is a very | ||
| 19 | * rare operation. When the Fence is not blocked, entering and leaving a | ||
| 20 | * critical region requires only a 1-2 atomic operations. In order to block | ||
| 21 | * the fence, no new threads can enter a critical region. When all entered | ||
| 22 | * regions are left, the fence is blocked. Waiting is done through slow busy | ||
| 23 | * wait. | ||
| 24 | */ | ||
| 25 | class Fence : public SingleCopy { | ||
| 26 | FRIEND_TEST(T_Fence, Basics); | ||
| 27 | |||
| 28 | public: | ||
| 29 | 14 | Fence() { | |
| 30 | 14 | atomic_init64(&counter_); | |
| 31 | 14 | atomic_init32(&blocking_); | |
| 32 | 14 | } | |
| 33 | |||
| 34 | 28 | void Enter() { | |
| 35 |
1/2✗ Branch 1 not taken.
✓ Branch 2 taken 28 times.
|
28 | while (atomic_read32(&blocking_)) { |
| 36 | ✗ | SafeSleepMs(kBusyWaitBackoffMs); | |
| 37 | } | ||
| 38 | 28 | atomic_inc64(&counter_); | |
| 39 | 28 | } | |
| 40 | |||
| 41 | 28 | void Leave() { atomic_dec64(&counter_); } | |
| 42 | |||
| 43 | 28 | void Close() { atomic_cas32(&blocking_, 0, 1); } | |
| 44 | |||
| 45 | /** | ||
| 46 | * Close and let live critical regions exit | ||
| 47 | */ | ||
| 48 | 14 | void Drain() { | |
| 49 | 14 | Close(); | |
| 50 |
1/2✗ Branch 1 not taken.
✓ Branch 2 taken 14 times.
|
14 | while (atomic_read64(&counter_) > 0) { |
| 51 | ✗ | SafeSleepMs(kBusyWaitBackoffMs); | |
| 52 | } | ||
| 53 | 14 | } | |
| 54 | |||
| 55 | 28 | void Open() { atomic_cas32(&blocking_, 1, 0); } | |
| 56 | |||
| 57 | private: | ||
| 58 | static const unsigned kBusyWaitBackoffMs = 100; | ||
| 59 | |||
| 60 | /** | ||
| 61 | * Number of active critical regions. | ||
| 62 | */ | ||
| 63 | atomic_int64 counter_; | ||
| 64 | |||
| 65 | /** | ||
| 66 | * A boolean that indicates if the fence is blocked. | ||
| 67 | */ | ||
| 68 | atomic_int32 blocking_; | ||
| 69 | }; | ||
| 70 | |||
| 71 | |||
| 72 | /** | ||
| 73 | * RAII wrapper in case an entire function or code block should be protected | ||
| 74 | * by a fence. | ||
| 75 | */ | ||
| 76 | class FenceGuard { | ||
| 77 | public: | ||
| 78 | 14 | explicit FenceGuard(Fence *fence) : fence_(fence) { fence_->Enter(); } | |
| 79 | 14 | ~FenceGuard() { fence_->Leave(); } | |
| 80 | |||
| 81 | private: | ||
| 82 | Fence *fence_; | ||
| 83 | }; | ||
| 84 | |||
| 85 | #ifdef CVMFS_NAMESPACE_GUARD | ||
| 86 | } // namespace CVMFS_NAMESPACE_GUARD | ||
| 87 | #endif | ||
| 88 | |||
| 89 | #endif // CVMFS_FENCE_H_ | ||
| 90 |