CPython: Fix Assertion Failure In Profiling.sampling
Bug Description
Guys, we've got a bit of a tricky bug here in CPython's profiling.sampling module. It seems like under certain conditions, it can trigger an assertion failure, specifically !(has_gil && gil_requested). This basically means the code is getting confused about whether it holds the Global Interpreter Lock (GIL) or has requested it. This assertion is designed to catch race conditions in GIL state tracking, but it's firing when it shouldn't, indicating a potential synchronization issue.
Understanding the GIL: The Global Interpreter Lock (GIL) is a mutex that protects access to Python objects, preventing multiple threads from executing Python bytecode at once. This simplifies memory management and avoids certain race conditions but can also limit the performance of multi-threaded CPU-bound programs. Properly managing the GIL is crucial for the stability and correctness of CPython.
The Assertion: The failing assertion, !(has_gil && gil_requested), checks that the thread doesn't simultaneously hold the GIL (has_gil) and have a pending request for it (gil_requested). This condition should never occur, and its presence suggests a flaw in the GIL's acquisition or release logic.
Why This Matters: Assertion failures are serious. They indicate a fundamental inconsistency in the program's state and can lead to unpredictable behavior, including crashes. Addressing this issue is vital for ensuring the reliability of CPython, especially in scenarios where profiling and sampling are used extensively for performance analysis and debugging.
Reproducing the Bug
Okay, so here's how you can try to reproduce this issue. First, you'll need a debug build of CPython. You can configure this using the --with-pydebug flag:
./configure --with-pydebug
make -j8
./python -m profiling.sampling run --live -i 1 -d 100000 -a -m test
# waiting but not 100% reproducer
This might take a bit to trigger, but there's a more reliable way. By adding a small delay in ceval_gil.c, we can widen the window for the race condition to occur:
diff --git a/Python/ceval_gil.c b/Python/ceval_gil.c
index f6ada3892f8..a5b1290ac70 100644
--- a/Python/ceval_gil.c
+++ b/Python/ceval_gil.c
@@ -409,9 +409,9 @@ take_gil(PyThreadState *tstate)
_PyThreadState_HangThread(tstate);
}
assert(_PyThreadState_CheckConsistency(tstate));
-
- tstate->gil_requested = 0;
tstate->holds_gil = 1;
+ for (volatile int _i = 0; _i < 10; _i++) {} // Widen window
+ tstate->gil_requested = 0;
_Py_unset_eval_breaker_bit(tstate, _PY_GIL_DROP_REQUEST_BIT);
update_eval_breaker_for_thread(interp, tstate);
With that change, you should be able to reproduce the bug more consistently using:
./python -m profiling.sampling run --live -i 1 -d 100000 -a -m test
The Root Cause
So, what's actually going on here? It seems like the issue arises because of the interaction between remote debugging and GIL management. When you're remotely debugging, things get a bit more complicated, especially when threads are involved. The debugger needs to pause and inspect threads, which can interfere with the normal GIL acquisition and release process. This interference, combined with the timing of the profiling.sampling module, can lead to the assertion failure.
The core problem is a potential race condition where the thread state isn't being updated atomically with respect to the GIL's state. This can happen when a thread is interrupted or paused during a critical section related to GIL management. Specifically, the debugger might be interfering with the setting or clearing of the gil_requested flag, leading to the inconsistent state that triggers the assertion.
Remote Debugging Challenges: Remote debugging introduces additional complexity because it involves pausing and inspecting threads from a separate process. This can disrupt the timing and execution flow of the target program, potentially exposing race conditions that would not occur under normal circumstances.
The Timing Factor: The profiling.sampling module likely exacerbates the issue because it introduces its own timing-sensitive operations. By periodically sampling the execution state of threads, it increases the likelihood of interrupting a thread at a critical point in its GIL management operations.
The Fix
Alright, so here's the fix. Since we can't easily modify ceval_gil.c in a remote debugging scenario, the best approach is to drop the problematic assertion in Modules/_remote_debugging/threads.c:
diff --git a/Modules/_remote_debugging/threads.c b/Modules/_remote_debugging/threads.c
index 99147b01a1b..69a85761562 100644
--- a/Modules/_remote_debugging/threads.c
+++ b/Modules/_remote_debugging/threads.c
@@ -335,12 +335,9 @@ unwind_stack_for_thread(
#endif
if (has_gil) {
status_flags |= THREAD_STATUS_HAS_GIL;
+ gil_requested = 0;
}
- // Assert that we never have both HAS_GIL and GIL_REQUESTED set at the same time
- // This would indicate a race condition in the GIL state tracking
- assert(!(has_gil && gil_requested));
-
// Check CPU status
long pthread_id = GET_MEMBER(long, ts, unwinder->debug_offsets.thread_state.thread_id);
Why This Works: By removing the assertion, we're essentially telling the debugger to ignore this particular race condition. Now, I know what you're thinking: "Isn't that just sweeping the problem under the rug?" Well, yes and no. The underlying race condition might still exist, but it's not causing any actual harm. The assertion is just being overly sensitive and firing when it shouldn't.
Safety Considerations: It's crucial to recognize that removing an assertion can mask genuine issues. However, in this context, the assertion seems to be a case of overzealous error detection. The fact that the race condition only manifests during remote debugging, and doesn't lead to any observable errors or crashes, suggests that it's benign.
Explanation of the fix
This patch modifies Modules/_remote_debugging/threads.c to remove an assertion that checks for a specific condition related to the Global Interpreter Lock (GIL) state. Let's break down the code and the reasoning behind this change.
Code Context:
The code snippet is part of the unwind_stack_for_thread function, which is involved in inspecting the state of a thread during remote debugging. This function is called to collect information about the thread's stack and status.
Original Assertion:
The original code included the following assertion:
assert(!(has_gil && gil_requested));
This assertion checks that the thread does not simultaneously hold the GIL (has_gil) and have a pending request for it (gil_requested). The intention is to catch potential race conditions in the GIL state tracking.
Problem:
The assertion was found to fail under certain conditions during remote debugging, specifically when using the profiling.sampling module. This indicates that the GIL state tracking in the remote debugging context is not perfectly synchronized, leading to a situation where the assertion is triggered even though there is no actual error.
Why the Assertion Fails:
The failure of the assertion is likely due to the asynchronous nature of remote debugging. When a debugger pauses a thread, it can interrupt the thread's execution at any point, including during GIL acquisition or release. This can lead to a situation where the has_gil and gil_requested flags are not updated atomically, causing the assertion to fail.
The Fix:
The fix is to remove the assertion:
--- a/Modules/_remote_debugging/threads.c
+++ b/Modules/_remote_debugging/threads.c
@@ -335,12 +335,9 @@ unwind_stack_for_thread(
#endif
if (has_gil) {
status_flags |= THREAD_STATUS_HAS_GIL;
+ gil_requested = 0;
}
- // Assert that we never have both HAS_GIL and GIL_REQUESTED set at the same time
- // This would indicate a race condition in the GIL state tracking
- assert(!(has_gil && gil_requested));
-
// Check CPU status
long pthread_id = GET_MEMBER(long, ts, unwinder->debug_offsets.thread_state.thread_id);
By removing the assertion, the code no longer checks for this specific condition. This prevents the false positive assertion failure during remote debugging.
Rationale:
The rationale behind this fix is that the assertion is overly sensitive in the context of remote debugging. The condition it checks for is not necessarily indicative of a real error, but rather a consequence of the asynchronous nature of debugging.
Safety Considerations:
It's important to note that removing assertions should be done with caution. Assertions are there to catch potential errors and ensure the correctness of the code. However, in this case, the assertion was found to be a false positive, and removing it does not introduce any new risks.
Tested On
This fix has been tested on the CPython main branch and on Linux operating systems.