diff --git a/xwords4/common/xwmutex.c b/xwords4/common/xwmutex.c
index 730e56f60..38ce2329d 100644
--- a/xwords4/common/xwmutex.c
+++ b/xwords4/common/xwmutex.c
@@ -1,4 +1,4 @@
-/* -*-mode: C; fill-column: 78; c-basic-offset: 4; -*- */
+/* -*- compile-command: "cd ../linux && make MEMDEBUG=TRUE -j3"; -*- */
 /* 
  * Copyright 2024 by Eric House (xwords@eehouse.org).  All rights reserved.
  *
@@ -20,40 +20,90 @@
 #include "xwmutex.h"
 
 #ifdef DEBUG
+#include <unistd.h>
 
 #define WAIT_ALL_SECS 3
 
+/* What is this? It's my attempt to, on debug builds and for all or only
+   chosen mutexes, get an assertion when the app doesn't get a lock in the
+   specified number of seconds. It's complicated because I want it to run on
+   Android, which doesn't have pthread_cancel().
+
+   It works by spawining a thread immediately before calling
+   pthread_mutex_lock(). If the lock succeeds, a flag is set so that the
+   thread won't raise an alarm. If the thread wakes from its sleep to find
+   that that flag isn't yet set, assertion failure...
+
+   The tricky part is how to share state between the checker thread and its
+   parent (that's calling pthread_mutex_lock().) Normally the parent might
+   pass a pointer to something on the stack to the thread proc, but normally
+   the stack frame will be long gone while the thread proc still wants to use
+   it. So: the flag that the checker thread will check lives in the thread's
+   own frame. The closure it's passed by the parent lets it set a pointer to
+   that flag so the parent can see it (the closure struct being in the
+   caller's scope.) So that the parent doesn't get out of pthread_mutex_lock()
+   before that pointer is set up, it busy-waits on its being set. Sue me: it's
+   debug code, and only active when set up for a particular mutex.
+ */
+
 typedef struct _WaitState {
     const char* caller;
+    const char* file;
+    int lineNo;
     XP_U16 waitSecs;
+    XP_Bool* flagLoc;
 } WaitState;
 
 static void*
-checkLockProc(void* closure)
+checkLockProc( void* closure )
 {
     WaitState* wsp = (WaitState*)closure;
-    sleep( wsp->waitSecs );
-    XP_LOGFF( "failed to get mutex in %d secs (caller: %s())",
-              wsp->waitSecs, wsp->caller );
-    XP_ASSERT(0);
+    const XP_UCHAR* file = wsp->file;
+    const XP_UCHAR* caller = wsp->caller;
+    XP_U16 waitSecs = wsp->waitSecs;
+    int lineNo = wsp->lineNo;
+    XP_Bool setMe = XP_FALSE;   /* caller will change on success */
+    XP_ASSERT( !wsp->flagLoc );
+    wsp->flagLoc = &setMe;      /* tells busy-waiting caller to run */
+
+    sleep( waitSecs );
+    if ( !setMe ) {
+        XP_LOGFF( "failed to get mutex in %d secs (caller: %s(), line %d in %s)",
+                  waitSecs, caller, lineNo, file );
+        XP_ASSERT(0);
+    }
     return NULL;
 }
 
 void
-mtx_lock_prv( MutexState* state, XP_U16 waitSecs, const char* caller )
+mtx_lock_prv( MutexState* state, XP_U16 waitSecs,
+              const char* file, const char* caller, int lineNo )
 {
-    // XP_LOGFF( "(caller=%s, waitSecs=%d)", caller, waitSecs );
-    pthread_t waitThread;
+    if ( 0 == waitSecs ) {
+        waitSecs  =state->waitSecs;
+    }
+
     WaitState ws = {
-        .waitSecs = 0 < waitSecs ? waitSecs : state->waitSecs,
+        .waitSecs = waitSecs,
+        .file = file,
         .caller = caller,
+        .lineNo = lineNo,
     };
     if ( 0 < waitSecs ) {
+        XP_ASSERT( !ws.flagLoc );
+        pthread_t waitThread;
         (void)pthread_create( &waitThread, NULL, checkLockProc, &ws );
+        int count = 0;
+        while ( !ws.flagLoc ) {
+            usleep(50); /* wait for thread to start */
+            if ( 0 == (++count%10) ) {
+                XP_LOGFF( "count %d; flagLoc still null", count );
+            }
+        }
     }
     pthread_mutex_lock( &state->mutex );
-    if ( 0 < waitSecs ) {
-        (void)pthread_cancel(waitThread);
+    if ( 0 < ws.waitSecs ) {
+        *ws.flagLoc = XP_TRUE;
     }
 }
 
@@ -94,6 +144,7 @@ mtx_init_prv( MutexState* mutex, XP_Bool recursive
     }
 # endif
     mutex->waitSecs = waitSecs;
+    XP_LOGFF( "set waitSecs: %d", mutex->waitSecs );
 #endif
     pthread_mutex_init( &mutex->mutex, &attr );
 #ifdef DEBUG
@@ -118,3 +169,35 @@ mtx_destroy_prv( MutexState* mutex )
 {
     pthread_mutex_destroy( &mutex->mutex );
 }
+
+#ifdef DEBUG
+static void*
+grabProc( void* closure )
+{
+    LOG_FUNC();
+    MutexState* ms = (MutexState*)closure;
+    WITH_MUTEX(ms);
+    sleep(20);
+    END_WITH_MUTEX();
+    LOG_RETURN_VOID();
+    return NULL;
+}
+
+void
+mtx_crashToTest()
+{
+    LOG_FUNC();
+    MutexState state1;
+    MUTEX_INIT_CHECKED( &state1, XP_FALSE, 5 );
+
+    /* One thread to grab it for 20 seconds */
+    pthread_t thread1;
+    (void)pthread_create( &thread1, NULL, grabProc, &state1 );
+
+    /* Another to try to grab it */
+    pthread_t thread2;
+    (void)pthread_create( &thread2, NULL, grabProc, &state1 );
+
+    pthread_join( thread2, NULL );
+}
+#endif
diff --git a/xwords4/common/xwmutex.h b/xwords4/common/xwmutex.h
index 6df930e65..cb61247fa 100644
--- a/xwords4/common/xwmutex.h
+++ b/xwords4/common/xwmutex.h
@@ -25,12 +25,13 @@
 
 #ifdef DEBUG
 
-void mtx_lock_prv(MutexState* state, XP_U16 waitSecs, const char* caller);
+void mtx_lock_prv(MutexState* state, XP_U16 waitSecs, const char* file,
+                  const char* caller, int lineNo);
 void mtx_unlock_prv(MutexState* state, XP_U16 waitSecs, const char* caller);
 # define WITH_MUTEX_CHECKED(STATE, SECS) {        \
     MutexState* _state = (STATE);                \
     XP_U16 _waitSecs = (SECS);                   \
-    mtx_lock_prv(_state, _waitSecs, __func__)
+    mtx_lock_prv(_state, _waitSecs, __FILE__, __func__, __LINE__)
 # define WITH_MUTEX(STATE) WITH_MUTEX_CHECKED(STATE, 0)
 # define END_WITH_MUTEX() mtx_unlock_prv(_state, _waitSecs, __func__);  \
     }
@@ -51,9 +52,12 @@ void mtx_init_prv( MutexState* mutex, XP_Bool recursive
 void mtx_destroy_prv( MutexState* mutex );
 
 #ifdef DEBUG
+void mtx_crashToTest();
+
 # define MUTEX_INIT_CHECKED(STATE, RECURSIVE, WS) mtx_init_prv((STATE), (RECURSIVE), (WS))
 # define MUTEX_INIT(STATE, RECURSIVE) MUTEX_INIT_CHECKED(STATE, RECURSIVE, 0)
 #else
+# define mtx_crashToTest()
 # define MUTEX_INIT(STATE, RECURSIVE) mtx_init_prv((STATE), (RECURSIVE))
 # define MUTEX_INIT_CHECKED(STATE, RECURSIVE, WS) MUTEX_INIT((STATE), (RECURSIVE))
 #endif
diff --git a/xwords4/linux/linuxmain.c b/xwords4/linux/linuxmain.c
index 0a8dd7c69..44cb250c4 100644
--- a/xwords4/linux/linuxmain.c
+++ b/xwords4/linux/linuxmain.c
@@ -76,6 +76,7 @@
 #include "dictiter.h"
 #include "gsrcwrap.h"
 #include "dllist.h"
+#include "xwmutex.h"
 /* #include "commgr.h" */
 /* #include "compipe.h" */
 #include "memstream.h"
@@ -2686,6 +2687,7 @@ main( int argc, char** argv )
     XP_LOGFF( "%s starting; ptr size: %zu", argv[0], sizeof(argv) );
 
     testDLL();
+    // mtx_crashToTest();
     // return 0;
 
     int opt;