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MDEV-25404: ssux_lock_low: Introduce a separate writer mutex
Having both readers and writers use a single lock word in futex system calls caused performance regression compared to SRW_LOCK_DUMMY (mutex and 2 condition variables). A contributing factor is that we did not accurately keep track of the number of waiting threads and thus had to invoke system calls to wake up any waiting threads. SUX_LOCK_GENERIC: Renamed from SRW_LOCK_DUMMY. This is the original implementation, with rw_lock (std::atomic<uint32_t>), a mutex and two condition variables. Using a separate writer mutex (as described below) is not possible, because the mutex ownership in a buf_block_t::lock must be able to transfer from a write submitter thread to an I/O completion thread, and pthread_mutex_lock() may assume that the submitter thread is recursively acquiring the mutex that it already holds, while in reality the I/O completion thread is the real owner. POSIX does not define an interface for requesting a mutex to be non-recursive. On Microsoft Windows, srw_lock_low will remain a simple wrapper of SRWLOCK. On 32-bit Microsoft Windows, sizeof(SRWLOCK)=4 while sizeof(srw_lock_low)=8. On other platforms, srw_lock_low is an alias of ssux_lock_low, the Simple (non-recursive) Shared/Update/eXclusive lock. In the futex-based implementation of ssux_lock_low (Linux, OpenBSD, Microsoft Windows), we shall use a dedicated mutex for exclusive requests (writer), and have a WRITER flag in the 'readers' lock word to inform that a writer is holding the lock or waiting for the lock to be granted. When the WRITER flag is set, all lock requests must acquire the writer mutex. Normally, shared (S) lock requests simply perform a compare-and-swap on the 'readers' word. Update locks are implemented as a combination of writer mutex and a normal counter in the 'readers' lock word. The conflict between U and X locks is guaranteed by the writer mutex. Unlike SUX_LOCK_GENERIC, wr_u_downgrade() will not wake up any pending rd_lock() waits. They will wait until u_unlock() releases the writer mutex. The ssux_lock_low is always wrapped by sux_lock (with a recursion count of U and X locks), used for dict_index_t::lock and buf_block_t::lock. Their memory footprint for the futex-based implementation will increase by sizeof(srw_mutex), or 4 bytes. This change addresses a performance regression in read-only benchmarks, such as sysbench oltp_read_only. Also write performance was improved. On 32-bit Linux and OpenBSD, lock_sys_t::hash_table will allocate two hash table elements for each srw_lock (14 instead of 15 hash table cells per 64-byte cache line on IA-32). On Microsoft Windows, sizeof(SRWLOCK)==sizeof(void*) and there is no change. Reviewed by: Vladislav Vaintroub Tested by: Axel Schwenke and Vladislav Vaintroub
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