i#4219 xl8: Add an rstat on synch xl8 failures

Adds an rstat "synchs_not_at_safe_spot" measuring the count of
synch_with_thread attempts that had to be re-tried due to the target
being suspended at an un-translatable (i.e., unsafe) spot.

Adds a corresponding dr_stats_t field "synchs_not_at_safe_spot" for
extraction.

These will be useful for measuring the impact of clean calls and other
un-translatable places on detach and other synch operations.

Issue: #4219

core/globals.h

@@ -301,6 +301,11 @@ typedef struct _dr_stats_t {
     uint64 peak_num_threads;
     /** Accumulated total number of threads encountered by DR. */
     uint64 num_threads_created;
+    /**
+     * Thread synchronization attempts retried due to the target thread being at
+     * an un-translatable spot.
+     */
+    uint64 synchs_not_at_safe_spot;
 } dr_stats_t;
 
 /* DR_API EXPORT END */

core/lib/instrument_api.h

@@ -6382,8 +6382,8 @@ DR_API
 /**
  * Retrieves various statistics exported by DR as global, process-wide values.
  * The API is not thread-safe.
- * The caller is expected to pass a pointer to a valid, initialized dr_stats_t
- * value, with the size field set (see dr_stats_t).
+ * The caller is expected to pass a pointer to a valid, initialized #dr_stats_t
+ * value, with the size field set (see #dr_stats_t).
  * Returns false if stats are not enabled.
  */
 bool

core/lib/statsx.h

@@ -245,6 +245,7 @@ STATS_DEF("Cache consistency flushes that flushed nothing", num_empty_flushes)
 STATS_DEF("Cache consistency flushes via synchall", flush_synchall)
 STATS_DEF("Thread not translated in synchall flush (race)", flush_synchall_races)
 STATS_DEF("Thread not synched with in synchall flush", flush_synchall_fail)
+RSTATS_DEF("Synch attempt failure b/c not at safe spot", synchs_not_at_safe_spot)
 STATS_DEF("Cache consistency coarse units flushed", flush_coarse_units)
 STATS_DEF("Cache consistency persisted units flushed", flush_persisted_units)
 STATS_DEF("Cache consistency persisted flushed at unload", flush_persisted_unload)

core/synch.c

@@ -1064,6 +1064,8 @@ synch_with_thread(thread_id_t id, bool block, bool hold_initexit_lock,
                     my_id);
                 res = THREAD_SYNCH_RESULT_SUCCESS;
                 break;
+            } else {
+                RSTATS_INC(synchs_not_at_safe_spot);
             }
             if (!os_thread_resume(trec)) {
                 ASSERT_NOT_REACHED();

core/utils.c

@@ -4606,5 +4606,8 @@ stats_get_snapshot(dr_stats_t *drstats)
     }
     drstats->peak_num_threads = GLOBAL_STAT(peak_num_threads);
     drstats->num_threads_created = GLOBAL_STAT(num_threads_created);
+    if (drstats->size > offsetof(dr_stats_t, synchs_not_at_safe_spot)) {
+        drstats->synchs_not_at_safe_spot = GLOBAL_STAT(synchs_not_at_safe_spot);
+    }
     return true;
 }