The grlib in TivaWare has the following mutex function in widget.c. Isn't it possible it could fail due to an interrupt occurring after the ldrexb instruction but before the following cmp instruction?
#if defined(ewarm) || defined(DOXYGEN)
uint32_t
WidgetMutexGet(uint8_t *pi8Mutex)
{
//
// Acquire the mutex if possible.
//
__asm(" mov r1, #1\n"
" ldrexb r2, [r0]\n"
" cmp r2, #0\n"
" it eq\n"
" strexb r2, r1, [r0]\n"
" mov r0, r2\n");
//
// "Warning[Pe940]: missing return statement at end of non-void function"
// is suppressed here to avoid putting a "bx lr" in the inline assembly
// above and a superfluous return statement here.
//
#pragma diag_suppress=Pe940
}
#pragma diag_default=Pe940
#endif
