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b81382887c
In the SUX_LOCK_GENERIC implementation, we can remember at most one pending exclusive lock request. If multiple exclusive lock requests are pending, the WRITER_WAITING flag will be cleared when the first waiting writer acquires the exclusive lock. ssux_lock_low::update_lock(): If WRITER_WAITING is set, wake up the writer even if the UPDATER flag is set, because the waiting writer may be in the process of upgrading its U lock to X. rw_lock::read_unlock(): Also indicate that an X lock waiter must be woken up if an U lock exists. This fix may cause unnecessary wake-ups and system calls, but this is the best that we can do. Ideally we would use the MDEV-25404 idea of a separate 'writer' mutex, but there is no portable way to request that a non-recursive mutex be created, and InnoDB requires the ability to transfer buf_block_t::lock ownership to an I/O thread. To allow problems like this to be caught more reliably in the future, we add a unit test for srw_mutex, srw_lock, ssux_lock, sux_lock.
227 lines
8.1 KiB
C++
227 lines
8.1 KiB
C++
/*****************************************************************************
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Copyright (c) 2020, 2021, MariaDB Corporation.
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This program is free software; you can redistribute it and/or modify it under
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the terms of the GNU General Public License as published by the Free Software
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Foundation; version 2 of the License.
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This program is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
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You should have received a copy of the GNU General Public License along with
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this program; if not, write to the Free Software Foundation, Inc.,
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51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA
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*****************************************************************************/
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#pragma once
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#include <atomic>
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#include "my_dbug.h"
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#if !(defined __linux__ || defined __OpenBSD__ || defined _WIN32)
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# define SUX_LOCK_GENERIC
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#elif 0 // defined SAFE_MUTEX
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# define SUX_LOCK_GENERIC /* Use dummy implementation for debugging purposes */
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#endif
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#ifdef SUX_LOCK_GENERIC
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/** Simple read-update-write lock based on std::atomic */
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#else
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/** Simple read-write lock based on std::atomic */
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#endif
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class rw_lock
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{
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/** The lock word */
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std::atomic<uint32_t> lock;
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protected:
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/** Available lock */
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static constexpr uint32_t UNLOCKED= 0;
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/** Flag to indicate that write_lock() is being held */
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static constexpr uint32_t WRITER= 1U << 31;
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/** Flag to indicate that write_lock_wait() is pending */
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static constexpr uint32_t WRITER_WAITING= 1U << 30;
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/** Flag to indicate that write_lock() or write_lock_wait() is pending */
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static constexpr uint32_t WRITER_PENDING= WRITER | WRITER_WAITING;
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#ifdef SUX_LOCK_GENERIC
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/** Flag to indicate that an update lock exists */
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static constexpr uint32_t UPDATER= 1U << 29;
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#endif /* SUX_LOCK_GENERIC */
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/** Start waiting for an exclusive lock.
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@return current value of the lock word */
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uint32_t write_lock_wait_start()
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{ return lock.fetch_or(WRITER_WAITING, std::memory_order_relaxed); }
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/** Wait for an exclusive lock.
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@param l the value of the lock word
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@return whether the exclusive lock was acquired */
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bool write_lock_wait_try(uint32_t &l)
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{
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return lock.compare_exchange_strong(l, WRITER, std::memory_order_acquire,
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std::memory_order_relaxed);
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}
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/** Try to acquire a shared lock.
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@tparam prioritize_updater whether to ignore WRITER_WAITING for UPDATER
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@param l the value of the lock word
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@return whether the lock was acquired */
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#ifdef SUX_LOCK_GENERIC
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template<bool prioritize_updater= false>
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#endif /* SUX_LOCK_GENERIC */
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bool read_trylock(uint32_t &l)
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{
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l= UNLOCKED;
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while (!lock.compare_exchange_strong(l, l + 1, std::memory_order_acquire,
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std::memory_order_relaxed))
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{
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DBUG_ASSERT(!(WRITER & l) || !(~WRITER_PENDING & l));
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#ifdef SUX_LOCK_GENERIC
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DBUG_ASSERT((~(WRITER_PENDING | UPDATER) & l) < UPDATER);
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if (prioritize_updater
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? (WRITER & l) || ((WRITER_WAITING | UPDATER) & l) == WRITER_WAITING
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: (WRITER_PENDING & l))
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#else /* SUX_LOCK_GENERIC */
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if (l & WRITER_PENDING)
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#endif /* SUX_LOCK_GENERIC */
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return false;
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}
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return true;
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}
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#ifdef SUX_LOCK_GENERIC
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/** Try to acquire an update lock.
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@param l the value of the lock word
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@return whether the lock was acquired */
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bool update_trylock(uint32_t &l)
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{
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l= UNLOCKED;
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while (!lock.compare_exchange_strong(l, l | UPDATER,
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std::memory_order_acquire,
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std::memory_order_relaxed))
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{
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DBUG_ASSERT(!(WRITER & l) || !(~WRITER_PENDING & l));
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DBUG_ASSERT((~(WRITER_PENDING | UPDATER) & l) < UPDATER);
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if ((WRITER_PENDING | UPDATER) & l)
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return false;
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}
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return true;
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}
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/** Try to upgrade an update lock to an exclusive lock.
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@return whether the update lock was upgraded to exclusive */
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bool upgrade_trylock()
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{
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auto l= UPDATER;
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while (!lock.compare_exchange_strong(l, WRITER,
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std::memory_order_acquire,
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std::memory_order_relaxed))
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{
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/* Either conflicting (read) locks have been granted, or
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the WRITER_WAITING flag was set by some thread that is waiting
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to become WRITER. */
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DBUG_ASSERT(((WRITER | UPDATER) & l) == UPDATER);
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if (~(WRITER_WAITING | UPDATER) & l)
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return false;
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}
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DBUG_ASSERT((l & ~WRITER_WAITING) == UPDATER);
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/* Any thread that had set WRITER_WAITING will eventually be woken
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up by ssux_lock_low::x_unlock() or ssux_lock_low::u_unlock()
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(not ssux_lock_low::wr_u_downgrade() to keep the code simple). */
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return true;
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}
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/** Downgrade an exclusive lock to an update lock. */
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void downgrade()
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{
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IF_DBUG_ASSERT(auto l=,)
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lock.fetch_xor(WRITER | UPDATER, std::memory_order_relaxed);
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DBUG_ASSERT((l & ~WRITER_WAITING) == WRITER);
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}
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#endif /* SUX_LOCK_GENERIC */
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/** Wait for an exclusive lock.
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@return whether the exclusive lock was acquired */
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bool write_lock_poll()
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{
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auto l= WRITER_WAITING;
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if (write_lock_wait_try(l))
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return true;
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if (!(l & WRITER_WAITING))
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/* write_lock() must have succeeded for another thread */
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write_lock_wait_start();
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return false;
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}
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/** @return the lock word value */
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uint32_t value() const { return lock.load(std::memory_order_acquire); }
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public:
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/** Default constructor */
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rw_lock() : lock(UNLOCKED) {}
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/** Release a shared lock.
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@return whether any writers may have to be woken up */
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bool read_unlock()
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{
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auto l= lock.fetch_sub(1, std::memory_order_release);
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DBUG_ASSERT(!(l & WRITER)); /* no write lock must have existed */
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#ifdef SUX_LOCK_GENERIC
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DBUG_ASSERT(~(WRITER_PENDING | UPDATER) & l); /* at least one read lock */
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return (~(WRITER_PENDING | UPDATER) & l) == 1;
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#else /* SUX_LOCK_GENERIC */
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DBUG_ASSERT(~(WRITER_PENDING) & l); /* at least one read lock */
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return (~WRITER_PENDING & l) == 1;
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#endif /* SUX_LOCK_GENERIC */
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}
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#ifdef SUX_LOCK_GENERIC
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/** Release an update lock */
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void update_unlock()
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{
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IF_DBUG_ASSERT(auto l=,)
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lock.fetch_and(~UPDATER, std::memory_order_release);
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/* the update lock must have existed */
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DBUG_ASSERT((l & (WRITER | UPDATER)) == UPDATER);
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}
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#endif /* SUX_LOCK_GENERIC */
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/** Release an exclusive lock */
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void write_unlock()
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{
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IF_DBUG_ASSERT(auto l=,)
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lock.fetch_and(~WRITER, std::memory_order_release);
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/* the write lock must have existed */
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#ifdef SUX_LOCK_GENERIC
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DBUG_ASSERT((l & (WRITER | UPDATER)) == WRITER);
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#else /* SUX_LOCK_GENERIC */
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DBUG_ASSERT(l & WRITER);
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#endif /* SUX_LOCK_GENERIC */
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}
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/** Try to acquire a shared lock.
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@return whether the lock was acquired */
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bool read_trylock() { uint32_t l; return read_trylock(l); }
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/** Try to acquire an exclusive lock.
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@return whether the lock was acquired */
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bool write_trylock()
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{
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auto l= UNLOCKED;
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return lock.compare_exchange_strong(l, WRITER, std::memory_order_acquire,
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std::memory_order_relaxed);
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}
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/** @return whether an exclusive lock is being held by any thread */
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bool is_write_locked() const
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{ return !!(lock.load(std::memory_order_relaxed) & WRITER); }
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#ifdef SUX_LOCK_GENERIC
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/** @return whether an update lock is being held by any thread */
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bool is_update_locked() const
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{ return !!(lock.load(std::memory_order_relaxed) & UPDATER); }
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#endif /* SUX_LOCK_GENERIC */
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/** @return whether a shared lock is being held by any thread */
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bool is_read_locked() const
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{
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auto l= lock.load(std::memory_order_relaxed);
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return (l & ~WRITER_PENDING) && !(l & WRITER);
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}
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/** @return whether any lock is being held or waited for by any thread */
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bool is_locked_or_waiting() const
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{ return lock.load(std::memory_order_relaxed) != 0; }
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/** @return whether any lock is being held by any thread */
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bool is_locked() const
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{ return (lock.load(std::memory_order_relaxed) & ~WRITER_WAITING) != 0; }
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};
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