Hello, Michael Kelly, le mar. 23 sept. 2025 15:26:45 +0100, a ecrit: > 3) thread0 in the worker generates a page fault. This causes page-in > involving a (top) vm_object which also has a shadow object. A mapping is > made between the top object/offset and a fictitious page to block other > threads from attempting the same page-in until thread0 has handled the page > fault. thread0 then traverses the object chain to the shadow and makes the > memory_object_data_request on the shadow object/offset and blocks itself > until the reply has arrived and been processed. > > 4) A signal is received by the process and is handled by (say) thread1. As > per normal signal handling, this results in thread0 being suspended by > thread1 via the system call to thread_suspend(). It can be immediately > suspended because thread0 is in TH_WAIT state and is interruptible (TH_UNINT > not set).
Ah, the suspension also holds any kernel activity of thread0, so it won't be able to do what it promised? (paging in) > 5) After thread1 has suspended thread0 it trips a page fault itself which > actually requires the same page that was being paged-in by thread0. thread1 > now blocks indefinitely and cannot proceed until the original page-in > completes which of course it cannot as thread0 is suspended. thread0 will > only be resumed by thread1 and thread1 cannot continue because of the state > managed by thread0. > > I have some confidence that the above sequence is broadly what is happening > but it's difficult to be certain. I've got to this stage by adding extra > state to data structures rather than the otherwise huge volume of debug > logging which normally alters the timing to the point of masking the problem > anyway. In any case, I think that the scenario described above is possible > and provides a good match against the evidence that I do have. > > I have some very vague ideas for solutions but before discussing those it > would be helpful to have my analysis scrutinised for obvious error. The scenario looks plausible to me indeed. Samuel
