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/*****************************************************************************
Copyright (c) 2020, 2022, 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-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;
/** Start waiting for an exclusive lock. */
void write_lock_wait_start()
{
#if defined __GNUC__ && (defined __i386__ || defined __x86_64__)
static_assert(WRITER_WAITING == 1U << 30, "compatibility");
__asm__ __volatile__("lock btsl $30, %0" : "+m" (lock));
#elif defined _MSC_VER && (defined _M_IX86 || defined _M_X64)
static_assert(WRITER_WAITING == 1U << 30, "compatibility");
_interlockedbittestandset(reinterpret_cast<volatile long*>(&lock), 30);
#else
lock.fetch_or(WRITER_WAITING, std::memory_order_relaxed);
#endif
}
/** Start waiting for an exclusive lock.
@return current value of the lock word */
uint32_t write_lock_wait_start_read()
{ 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.
@param l the value of the lock word
@return whether the lock was acquired */
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));
if (l & WRITER_PENDING)
return false;
}
return true;
}
/** 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(!(l & WRITER)); /* no write lock must have existed */
DBUG_ASSERT(~(WRITER_PENDING) & l); /* at least one read lock */
return (~WRITER_PENDING & l) == 1;
}
/** Release an exclusive lock */
void write_unlock()
{
/* Below, we use fetch_sub(WRITER) instead of fetch_and(~WRITER).
The reason is that on IA-32 and AMD64 it translates into the 80486
instruction LOCK XADD, while fetch_and() translates into a loop
around LOCK CMPXCHG. For other ISA either form should be fine. */
static_assert(WRITER == 1U << 31, "compatibility");
IF_DBUG_ASSERT(auto l=,) lock.fetch_sub(WRITER, std::memory_order_release);
/* the write lock must have existed */
DBUG_ASSERT(l & 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 !!(value() & WRITER); }
/** @return whether any lock is being held or waited for by any thread */
bool is_locked_or_waiting() const { return value() != 0; }
/** @return whether any lock is being held by any thread */
bool is_locked() const { return (value() & ~WRITER_WAITING) != 0; }
};
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