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/*****************************************************************************
Copyright (c) 2020, 2021, MariaDB Corporation.
This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; version 2 of the License.
This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA
*****************************************************************************/
#pragma once
#include <atomic>
#include "my_dbug.h"
/** Simple read-update-write lock based on std::atomic */
class rw_lock
{
/** The lock word */
std::atomic<uint32_t> lock;
protected:
/** Available lock */
static constexpr uint32_t UNLOCKED= 0;
/** Flag to indicate that write_lock() is being held */
static constexpr uint32_t WRITER= 1U << 31;
/** Flag to indicate that write_lock_wait() is pending */
static constexpr uint32_t WRITER_WAITING= 1U << 30;
/** Flag to indicate that write_lock() or write_lock_wait() is pending */
static constexpr uint32_t WRITER_PENDING= WRITER | WRITER_WAITING;
/** Flag to indicate that an update lock exists */
static constexpr uint32_t UPDATER= 1U << 29;
/** Start waiting for an exclusive lock.
@return current value of the lock word */
uint32_t write_lock_wait_start()
{ return lock.fetch_or(WRITER_WAITING, std::memory_order_relaxed); }
/** Wait for an exclusive lock.
@param l the value of the lock word
@return whether the exclusive lock was acquired */
bool write_lock_wait_try(uint32_t &l)
{
return lock.compare_exchange_strong(l, WRITER, std::memory_order_acquire,
std::memory_order_relaxed);
}
/** Try to acquire a shared lock.
@tparam prioritize_updater whether to ignore WRITER_WAITING for UPDATER
@param l the value of the lock word
@return whether the lock was acquired */
template<bool prioritize_updater= false>
bool read_trylock(uint32_t &l)
{
l= UNLOCKED;
while (!lock.compare_exchange_strong(l, l + 1, std::memory_order_acquire,
std::memory_order_relaxed))
{
DBUG_ASSERT(!(WRITER & l) || !(~WRITER_PENDING & l));
DBUG_ASSERT((~(WRITER_PENDING | UPDATER) & l) < UPDATER);
if (prioritize_updater
? (WRITER & l) || ((WRITER_WAITING | UPDATER) & l) == WRITER_WAITING
: (WRITER_PENDING & l))
return false;
}
return true;
}
/** Try to acquire an update lock.
@param l the value of the lock word
@return whether the lock was acquired */
bool update_trylock(uint32_t &l)
{
l= UNLOCKED;
while (!lock.compare_exchange_strong(l, l | UPDATER,
std::memory_order_acquire,
std::memory_order_relaxed))
{
DBUG_ASSERT(!(WRITER & l) || !(~WRITER_PENDING & l));
DBUG_ASSERT((~(WRITER_PENDING | UPDATER) & l) < UPDATER);
if ((WRITER_PENDING | UPDATER) & l)
return false;
}
return true;
}
/** Try to upgrade an update lock to an exclusive lock.
@return whether the update lock was upgraded to exclusive */
bool upgrade_trylock()
{
auto l= UPDATER;
while (!lock.compare_exchange_strong(l, WRITER,
std::memory_order_acquire,
std::memory_order_relaxed))
{
/* Either conflicting (read) locks have been granted, or
the WRITER_WAITING flag was set by some thread that is waiting
to become WRITER. */
DBUG_ASSERT(((WRITER | UPDATER) & l) == UPDATER);
if (~(WRITER_WAITING | UPDATER) & l)
return false;
}
DBUG_ASSERT((l & ~WRITER_WAITING) == UPDATER);
/* Any thread that had set WRITER_WAITING will eventually be woken
up by ssux_lock_low::x_unlock() or ssux_lock_low::u_unlock()
(not ssux_lock_low::wr_u_downgrade() to keep the code simple). */
return true;
}
/** Downgrade an exclusive lock to an update lock. */
void downgrade()
{
IF_DBUG_ASSERT(auto l=,)
lock.fetch_xor(WRITER | UPDATER, std::memory_order_relaxed);
DBUG_ASSERT((l & ~WRITER_WAITING) == WRITER);
}
/** Wait for an exclusive lock.
@return whether the exclusive lock was acquired */
bool write_lock_poll()
{
auto l= WRITER_WAITING;
if (write_lock_wait_try(l))
return true;
if (!(l & WRITER_WAITING))
/* write_lock() must have succeeded for another thread */
write_lock_wait_start();
return false;
}
/** @return the lock word value */
uint32_t value() const { return lock.load(std::memory_order_acquire); }
public:
/** Default constructor */
rw_lock() : lock(UNLOCKED) {}
/** Release a shared lock.
@return whether any writers may have to be woken up */
bool read_unlock()
{
auto l= lock.fetch_sub(1, std::memory_order_release);
DBUG_ASSERT(~(WRITER_PENDING | UPDATER) & l); /* at least one read lock */
DBUG_ASSERT(!(l & WRITER)); /* no write lock must have existed */
return (~WRITER_PENDING & l) == 1;
}
/** Release an update lock */
void update_unlock()
{
IF_DBUG_ASSERT(auto l=,)
lock.fetch_and(~UPDATER, std::memory_order_release);
/* the update lock must have existed */
DBUG_ASSERT((l & (WRITER | UPDATER)) == UPDATER);
}
/** Release an exclusive lock */
void write_unlock()
{
IF_DBUG_ASSERT(auto l=,)
lock.fetch_and(~WRITER, std::memory_order_release);
/* the write lock must have existed */
DBUG_ASSERT((l & (WRITER | UPDATER)) == WRITER);
}
/** Try to acquire a shared lock.
@return whether the lock was acquired */
bool read_trylock() { uint32_t l; return read_trylock(l); }
/** Try to acquire an exclusive lock.
@return whether the lock was acquired */
bool write_trylock()
{
auto l= UNLOCKED;
return lock.compare_exchange_strong(l, WRITER, std::memory_order_acquire,
std::memory_order_relaxed);
}
/** @return whether an exclusive lock is being held by any thread */
bool is_write_locked() const
{ return !!(lock.load(std::memory_order_relaxed) & WRITER); }
/** @return whether an update lock is being held by any thread */
bool is_update_locked() const
{ return !!(lock.load(std::memory_order_relaxed) & UPDATER); }
/** @return whether a shared lock is being held by any thread */
bool is_read_locked() const
{
auto l= lock.load(std::memory_order_relaxed);
return (l & ~WRITER_PENDING) && !(l & WRITER);
}
/** @return whether any lock is being held or waited for by any thread */
bool is_locked_or_waiting() const
{ return lock.load(std::memory_order_relaxed) != 0; }
/** @return whether any lock is being held by any thread */
bool is_locked() const
{ return (lock.load(std::memory_order_relaxed) & ~WRITER_WAITING) != 0; }
};
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