The implementation of Python thread local variables (threading.local() or _thread._local) has some thread-safety issues. The issues are particularly relevant to the free-threaded build, but some can affect the default build too.
local_clear loop over threads isn't safe
The local_clear() function is called when a thread local variable is destroyed. It loops over all threads in the interpreter and removes itself from their dictionaries.
This isn't thread-safe because after HEAD_UNLOCK(runtime), the stored tstate might be deleted concurrently. This can happen even in the default build with the GIL enabled because PyThreadState_Delete() doesn't require the GIL to be held. However, it's less likely to occur in practice because threading module created threads hold onto the GIL until they're deleted.
local_clear access to tstate->dict isn't thread-safe
In the free-threaded build, local_clear() may be run concurrently with some other thread's PyThreadState_Clear(). The access to another thread's tstate->dict isn't safe because it may be getting destroyed concurrently.
Bug report
The implementation of Python thread local variables (
threading.local()
or_thread._local
) has some thread-safety issues. The issues are particularly relevant to the free-threaded build, but some can affect the default build too.local_clear loop over threads isn't safe
The
local_clear()
function is called when a thread local variable is destroyed. It loops over all threads in the interpreter and removes itself from their dictionaries.https://github.com/python/cpython/blob/fd0f814ade43fa479bfbe76dc226424db14a9354/Modules/_threadmodule.c#L1568-L1581
This isn't thread-safe because after
HEAD_UNLOCK(runtime)
, the storedtstate
might be deleted concurrently. This can happen even in the default build with the GIL enabled becausePyThreadState_Delete()
doesn't require the GIL to be held. However, it's less likely to occur in practice becausethreading
module created threads hold onto the GIL until they're deleted.local_clear access to
tstate->dict
isn't thread-safeIn the free-threaded build,
local_clear()
may be run concurrently with some other thread'sPyThreadState_Clear()
. The access to another thread'ststate->dict
isn't safe because it may be getting destroyed concurrently.