Revert "[3.15] gh-155811: Add a seqcount to gc_stats to prevent torn reads (GH-155828) (#158829)"

This reverts commit 0281240.

Author: pablogsal

Include/internal/pycore_interp_structs.h

@@ -219,7 +219,6 @@ struct gc_old_stats_buffer {
 struct gc_stats {
     struct gc_young_stats_buffer young;
     struct gc_old_stats_buffer old[2];
-    uint32_t update_seq;
 };
 
 struct _gc_runtime_state {

Misc/NEWS.d/next/Library/2026-08-15-10-20-40.gh-issue-155811.knP-YB.rst

@@ -103,42 +103,12 @@ get_gc_stats_from_interpreter_state(RuntimeOffsets *offsets,
     }
 
     struct gc_stats stats;
-    uintptr_t sequence_address = gc_stats_addr
-        + offsetof(struct gc_stats, update_seq);
-    /* A short GC update may finish before a second attempt. */
-    for (int attempt = 0; attempt < 2; attempt++) {
-        uint32_t before;
-        if (_Py_RemoteDebug_ReadRemoteMemory(&offsets->handle,
-                                             sequence_address,
-                                             sizeof(before), &before) < 0) {
-            set_exception_cause(offsets, PyExc_RuntimeError,
-                                "Failed to read GC update sequence");
-            return -1;
-        }
-        if (_Py_RemoteDebug_ReadRemoteMemory(&offsets->handle,
-                                             gc_stats_addr,
-                                             sizeof(stats),
-                                             &stats) < 0) {
-            set_exception_cause(offsets, PyExc_RuntimeError, "Failed to read GC state");
-            return -1;
-        }
-
-        uint32_t after;
-        if (_Py_RemoteDebug_ReadRemoteMemory(&offsets->handle,
-                                             sequence_address,
-                                             sizeof(after), &after) < 0) {
-            set_exception_cause(offsets, PyExc_RuntimeError,
-                                "Failed to read GC update sequence");
-            return -1;
-        }
-        if (before == after && before == stats.update_seq && !(after & 1)) {
-            break;
-        }
-        if (attempt == 1) {
-            PyErr_SetString(PyExc_RuntimeError,
-                            "GC stats changed while being read; retry later");
-            return -1;
-        }
+    if (_Py_RemoteDebug_ReadRemoteMemory(&offsets->handle,
+                                         gc_stats_addr,
+                                         sizeof(stats),
+                                         &stats) < 0) {
+        set_exception_cause(offsets, PyExc_RuntimeError, "Failed to read GC state");
+        return -1;
     }
 
     if (read_gc_stats(&stats, iid, ctx->result,

Modules/_remote_debugging/gc_stats.c

@@ -1399,13 +1399,6 @@ gc_get_prev_stats(GCState *gcstate, int gen)
 static void
 add_stats(GCState *gcstate, int gen, struct gc_generation_stats *stats)
 {
-    struct gc_stats *generation_stats = gcstate->generation_stats;
-    uint32_t seq = _Py_atomic_load_uint32_relaxed(&generation_stats->update_seq);
-    assert((seq & 1) == 0);
-    /* Odd seq tells the reader that an update is in progress. */
-    _Py_atomic_store_uint32_relaxed(&generation_stats->update_seq, seq + 1);
-    _Py_atomic_fence_seq_cst();
-
     struct gc_generation_stats *prev_stats = gc_get_prev_stats(gcstate, gen);
     struct gc_generation_stats *cur_stats = gc_get_stats(gcstate, gen);
 
@@ -1419,8 +1412,9 @@ add_stats(GCState *gcstate, int gen, struct gc_generation_stats *stats)
 
     cur_stats->duration += stats->duration;
     cur_stats->heap_size = stats->heap_size;
+    /* Publish ts_stop last so remote readers do not select a partially
+       updated stats record as the latest collection. */
     cur_stats->ts_stop = stats->ts_stop;
-    _Py_atomic_store_uint32_release(&generation_stats->update_seq, seq + 2);
 }
 
 /* This is the main function.  Read this to understand how the

Python/gc.c

@@ -2282,12 +2282,6 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
     }
 
     /* Update stats */
-    struct gc_stats *generation_stats = gcstate->generation_stats;
-    uint32_t seq = _Py_atomic_load_uint32_relaxed(&generation_stats->update_seq);
-    assert((seq & 1) == 0);
-    /* Odd seq tells the reader that an update is in progress. */
-    _Py_atomic_store_uint32_relaxed(&generation_stats->update_seq, seq + 1);
-    _Py_atomic_fence_seq_cst();
     struct gc_generation_stats *stats = get_stats(gcstate, generation);
     stats->ts_start = start;
     stats->ts_stop = stop;
@@ -2296,7 +2290,6 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
     stats->uncollectable += n;
     stats->duration += duration;
     stats->candidates += state.candidates;
-    _Py_atomic_store_uint32_release(&generation_stats->update_seq, seq + 2);
 
     GC_STAT_ADD(generation, objects_collected, m);
 #ifdef Py_STATS