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Rwlock
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| /** | ||
| * @file RWLock.h | ||
| * | ||
| * @section LICENSE | ||
| * | ||
| * The MIT License | ||
| * | ||
| * @copyright Copyright (c) 2017 Intel Corporation | ||
| * | ||
| * Permission is hereby granted, free of charge, to any person obtaining a copy | ||
| * of this software and associated documentation files (the "Software"), | ||
| * to deal in the Software without restriction, | ||
| * including without limitation the rights to use, copy, modify, | ||
| * merge, publish, distribute, sublicense, and/or sell | ||
| * copies of the Software, and to permit persons to whom the Software is | ||
| * furnished to do so, subject to the following conditions: | ||
| * | ||
| * The above copyright notice and this permission notice shall be included in | ||
| * all copies or substantial portions of the Software. | ||
| * | ||
| * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
| * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
| * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
| * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
| * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, | ||
| * ARISING FROM, | ||
| * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
| * THE SOFTWARE. | ||
| * | ||
| */ | ||
|
|
||
| #pragma once | ||
| #include <thread> | ||
| #include <random> | ||
| #include <vector> | ||
| #include <algorithm> | ||
|
|
||
| #include "arch.h" | ||
| #include "ExceptionsCommand.h" | ||
|
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| // All locking structures live in DRAM. We use stripe locks | ||
| // to balance out the space used by locks for a large database | ||
| // vs. parallelism. | ||
| namespace VDMS { | ||
| class RWLock | ||
| { | ||
| static const uint16_t LOCK_READER_MASK = 0x7fff; | ||
| static const uint16_t READER_INCR = 1; | ||
| static const uint16_t WRITER_LOCK_BIT = 15; | ||
| static const uint16_t WRITE_LOCK = 1UL << WRITER_LOCK_BIT; | ||
|
|
||
| // Backoff variables. | ||
| // *** Tune experimentally | ||
| static const size_t MIN_BACKOFF_DELAY = 100000; | ||
| static const size_t MAX_BACKOFF_DELAY = 50000000; | ||
| static const unsigned MAX_ATTEMPTS = 10; | ||
|
|
||
| uint16_t xadd(volatile uint16_t &m, uint16_t v) | ||
| { return ::xadd<uint16_t>(m, v); } | ||
| void atomic_and(volatile uint16_t &m, uint16_t v) | ||
| { ::atomic_and<uint16_t>(m, v); } | ||
|
|
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| volatile uint16_t _rw_lock; | ||
|
|
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| // Ideas from here: https://geidav.wordpress.com/tag/exponential-back-off | ||
| void backoff(size_t &cur_max_delay) | ||
| { | ||
| std::random_device rd; | ||
| thread_local std::uniform_int_distribution<size_t> dist; | ||
|
|
||
| // The simplest pseudo-random number generator that gives an int. | ||
| thread_local std::minstd_rand gen(rd()); | ||
|
|
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| size_t delay = cur_max_delay; | ||
| size_t temp = 2 * cur_max_delay; | ||
| cur_max_delay = (temp < MAX_BACKOFF_DELAY) ? temp : MAX_BACKOFF_DELAY; | ||
| const size_t count = dist(gen, decltype(dist)::param_type{delay, cur_max_delay}); | ||
| for (size_t i = 0; i < count; ++i) | ||
| pause(); | ||
| } | ||
|
|
||
| public: | ||
| RWLock() : _rw_lock(0) {} | ||
|
|
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| void read_lock() | ||
| { | ||
| size_t cur_max_delay = MIN_BACKOFF_DELAY; | ||
| unsigned attempts = 0; | ||
|
|
||
| while (1) { | ||
| uint16_t r = xadd(_rw_lock, READER_INCR); | ||
| if((r & LOCK_READER_MASK) == LOCK_READER_MASK) // if r was already maxed out | ||
| throw ExceptionCommand(LockError); // distinguish this from timeout. | ||
|
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||
| // Check if we get lock w/o any active writers | ||
| if ((r & WRITE_LOCK) == 0) | ||
| return; | ||
| xadd(_rw_lock, -READER_INCR); | ||
|
|
||
| // Wait for any active writers | ||
| while (_rw_lock & WRITE_LOCK) { | ||
| if (++attempts > MAX_ATTEMPTS) | ||
| throw ExceptionCommand(LockTimeout); | ||
| backoff(cur_max_delay); | ||
| } | ||
| } | ||
| } | ||
|
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| void read_unlock() | ||
| { | ||
| if ((_rw_lock & LOCK_READER_MASK) == 0) | ||
| throw ExceptionCommand(LockError); // distinguish this from timeout. | ||
| xadd(_rw_lock, -READER_INCR); | ||
| } | ||
|
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| void write_lock() | ||
| { | ||
| size_t cur_max_delay = MIN_BACKOFF_DELAY; | ||
| unsigned attempts = 0; | ||
|
|
||
| while (1) { | ||
| // Check if we get lock w/o any active writers | ||
| if (bts(_rw_lock, WRITER_LOCK_BIT) == 0) { | ||
| attempts = 0; | ||
|
|
||
| // Wait for any active readers | ||
| while(_rw_lock & LOCK_READER_MASK) { | ||
| if (++attempts > MAX_ATTEMPTS) { | ||
| atomic_and(_rw_lock, LOCK_READER_MASK); | ||
| throw ExceptionCommand(LockTimeout); | ||
| } | ||
| backoff(cur_max_delay); | ||
| } | ||
| return; | ||
| } | ||
|
|
||
| // Wait for any active writers | ||
| while (_rw_lock & WRITE_LOCK) { | ||
| if (++attempts > MAX_ATTEMPTS) { | ||
| throw ExceptionCommand(LockTimeout); | ||
| } | ||
| backoff(cur_max_delay); | ||
| } | ||
| } | ||
| } | ||
|
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||
| // This function should be called only when the caller already possesses | ||
| // a read lock. Rather than making this recursive, we are relying on the | ||
| // caller to use this correctly. | ||
| void upgrade_write_lock() | ||
| { | ||
| size_t cur_max_delay = MIN_BACKOFF_DELAY; | ||
| unsigned attempts = 0; | ||
|
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| while (1) { | ||
| // Check if we get lock w/o any active writers | ||
| if (bts(_rw_lock, WRITER_LOCK_BIT) == 0) { | ||
| attempts = 0; | ||
|
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| // Wait for any active readers | ||
| while ((_rw_lock & LOCK_READER_MASK) > 1) { | ||
| if (++attempts > MAX_ATTEMPTS) { | ||
| atomic_and(_rw_lock, LOCK_READER_MASK); | ||
| throw ExceptionCommand(LockTimeout); | ||
| } | ||
| backoff(cur_max_delay); | ||
| } | ||
|
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| // Don't need reader lock anymore | ||
| xadd(_rw_lock, -READER_INCR); | ||
| return; | ||
| } | ||
|
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| // Wait for any active writers | ||
| // Give this another extra attempt | ||
| while (_rw_lock & WRITE_LOCK) { | ||
| if (attempts++ > MAX_ATTEMPTS) { | ||
| throw ExceptionCommand(LockTimeout); | ||
| } | ||
| backoff(cur_max_delay); | ||
| } | ||
| } | ||
| } | ||
|
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| void write_unlock() | ||
| { | ||
| if((_rw_lock & WRITE_LOCK) == 0) | ||
| throw ExceptionCommand(LockError); // distinguish this from timeout. | ||
| atomic_and(_rw_lock, LOCK_READER_MASK); | ||
| } | ||
|
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| bool is_write_locked() { return _rw_lock & WRITE_LOCK; } | ||
|
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| uint16_t reader_count() const { return _rw_lock & LOCK_READER_MASK; } | ||
| }; | ||
| } |
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