This is a C-level implementation of a fast, re-entrant, optimistic lock for CPython. It is written in Cython. Under normal conditions, it is about 10x faster than threading.RLock because it avoids all locking unless two or more threads try to acquire it at the same time. Under congestion, it is still about 10% faster than RLock due to being implemented in Cython.
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from cpython cimport pythread from cpython.exc cimport PyErr_NoMemory cdef class FastRLock: """Fast, re-entrant locking. Under uncongested conditions, the lock is never acquired but only counted. Only when a second thread comes in and notices that the lock is needed, it acquires the lock and notifies the first thread to release it when it's done. This is all made possible by the wonderful GIL. """ cdef pythread.PyThread_type_lock _real_lock cdef long _owner # ID of thread owning the lock cdef int _count # re-entry count cdef int _pending_requests # number of pending requests for real lock cdef bint _is_locked # whether the real lock is acquired def __cinit__(self): self._owner = -1 self._count = 0 self._is_locked = False self._pending_requests = 0 self._real_lock = pythread.PyThread_allocate_lock() if self._real_lock is NULL: PyErr_NoMemory() def __dealloc__(self): if self._real_lock is not NULL: pythread.PyThread_free_lock(self._real_lock) self._real_lock = NULL def acquire(self, bint blocking=True): return lock_lock(self, pythread.PyThread_get_thread_ident(), blocking) def release(self): if self._owner != pythread.PyThread_get_thread_ident(): raise RuntimeError("cannot release un-acquired lock") unlock_lock(self) # compatibility with threading.RLock def __enter__(self): # self.acquire() return lock_lock(self, pythread.PyThread_get_thread_ident(), True) def __exit__(self, t, v, tb): # self.release() if self._owner != pythread.PyThread_get_thread_ident(): raise RuntimeError("cannot release un-acquired lock") unlock_lock(self) def _is_owned(self): return self._owner == pythread.PyThread_get_thread_ident() cdef inline bint lock_lock(FastRLock lock, long current_thread, bint blocking) nogil: # Note that this function *must* hold the GIL when being called. # We just use 'nogil' in the signature to make sure that no Python # code execution slips in that might free the GIL if lock._count: # locked! - by myself? if current_thread == lock._owner: lock._count += 1 return 1 elif not lock._pending_requests: # not locked, not requested - go! lock._owner = current_thread lock._count = 1 return 1 # need to get the real lock return _acquire_lock( lock, current_thread, pythread.WAIT_LOCK if blocking else pythread.NOWAIT_LOCK) cdef bint _acquire_lock(FastRLock lock, long current_thread, int wait) nogil: # Note that this function *must* hold the GIL when being called. # We just use 'nogil' in the signature to make sure that no Python # code execution slips in that might free the GIL if not lock._is_locked and not lock._pending_requests: # someone owns it but didn't acquire the real lock - do that # now and tell the owner to release it when done. Note that we # do not release the GIL here as we must absolutely be the one # who acquires the lock now. if not pythread.PyThread_acquire_lock(lock._real_lock, wait): return 0 #assert not lock._is_locked lock._is_locked = True lock._pending_requests += 1 with nogil: # wait for the lock owning thread to release it locked = pythread.PyThread_acquire_lock(lock._real_lock, wait) lock._pending_requests -= 1 #assert not lock._is_locked #assert lock._count == 0 if not locked: return 0 lock._is_locked = True lock._owner = current_thread lock._count = 1 return 1 cdef inline void unlock_lock(FastRLock lock) nogil: # Note that this function *must* hold the GIL when being called. # We just use 'nogil' in the signature to make sure that no Python # code execution slips in that might free the GIL #assert lock._owner == pythread.PyThread_get_thread_ident() #assert lock._count > 0 lock._count -= 1 if lock._count == 0: lock._owner = -1 if lock._is_locked: pythread.PyThread_release_lock(lock._real_lock) lock._is_locked = False
The FastRLock implementation optimises for the non-congested case. It works by exploiting the availability of the GIL. Since it knows that it holds the GIL when the acquire()/release() methods are called, it can safely check the lock for being held by other threads and just count any re-entries as long as it is always the same thread that acquires it. This is a lot faster than actually acquiring the underlying lock.
When a second thread wants to acquire the lock as well, it first checks the lock count and finds out that the lock is already owned. If the underlying lock is also held by another thread already, it then just frees the GIL and asks for acquiring the lock, just like RLock does. If the underlying lock is not held, however, it acquires it immediately and basically hands over the ownership by telling the current owner to free it when it's done. Then, it falls back to the normal non-owner behaviour that asks for the lock and will eventually acquire it when it gets released. This makes sure that the real lock is only acquired when at least two threads want it.
All of these operations are basically atomic because any thread that modifies the lock state always holds the GIL. Note that the implementation must not call any Python code while handling the lock, as calling into Python may lead to a context switch which hands over the GIL to another thread and thus breaks atomicity. Therefore, the code misuses Cython's 'nogil' annotation to make sure that no Python code slips in accidentally.
Here are some timings for 1) five acquire-release cycles ('lock_unlock'), 2) five acquire calls followed by five release calls (nested locking, 'reentrant_lock_unlock'), 3) a mixed and partly nested sequence of acquire and release calls ('mixed_lock_unlock') and 4) five acquire-release cycles that do not block. All four are benchmarked for the single threaded case and the multi threaded case with 10 threads. I also tested it with 20 threads only to see that it then takes about twice the time. Note also that the congested case is substantially slower for both locks, so I only looped 1000x here to get useful timings instead of 100000x for the single threaded case.
Testing threading.RLock sequential (x100000): lock_unlock : 1.408 sec reentrant_lock_unlock : 1.089 sec mixed_lock_unlock : 1.212 sec lock_unlock_nonblocking : 1.415 sec threaded 10T (x1000): lock_unlock : 1.188 sec reentrant_lock_unlock : 1.039 sec mixed_lock_unlock : 1.068 sec lock_unlock_nonblocking : 1.199 sec Testing FastRLock sequential (x100000): lock_unlock : 0.122 sec reentrant_lock_unlock : 0.124 sec mixed_lock_unlock : 0.137 sec lock_unlock_nonblocking : 0.156 sec threaded 10T (x1000): lock_unlock : 0.911 sec reentrant_lock_unlock : 0.938 sec mixed_lock_unlock : 0.953 sec lock_unlock_nonblocking : 0.916 sec