On Fri, Apr 12, 2024 at 02:11:49PM -0400, Alan Stern wrote: > On Sat, Apr 13, 2024 at 12:26:07AM +0800, Sam Sun wrote: > > On Fri, Apr 12, 2024 at 10:40 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > > I suspect the usb_hub_to_struct_hub() call is racing with the > > > spinlock-protected region in hub_disconnect() (in hub.c). > > > > > > > If there is any other thing I could help, please let me know. > > > > > > Try the patch below. It should eliminate that race, which hopefully > > > will fix the problem. > > > I applied this patch and tried to execute several times, no more > > kernel core dump in my environment. I think this bug is fixed by the > > patch. But I do have one more question about it. Since it is a data > > race bug, it has reproducibility issues originally. How can I confirm > > if a racy bug is fixed by test? This kind of bug might still have a > > race window but is harder to trigger. Just curious, not for this > > patch. I think this patch eliminates the racy window. > > If you don't what what is racing, then testing cannot prove that a race > is eliminated. However, if you do know where a race occurs then it's > easy to see how mutual exclusion can prevent the race from happening. > > In this case the bug might have had a different cause, something other > than a race between usb_hub_to_struct_hub() and hub_disconnect(). If > that's so then testing this patch would not be a definite proof that the > bug is gone. But if that race _is_ the cause of the bug then this patch > will fix it -- you can see that just by reading the code with no need > for testing. In fact, there still might be a remaining bug, because even with this patch it is possible for usb_hub_to_struct_hub() to return NULL. You can test for this possibility by editing the disable_show() routine: Move the initializations of the local variables out of the declaration lines and into the main part of the routine, and add a delay (like msleep(1000)) before the call to usb_hub_to_struct_hub() -- this will make the bug a lot easier to trigger (you could even do it by hand). I think to fully fix this problem it will be necessary to test whether hub is NULL before using it. The same problem ought to exist in disable_show(), even though your testing didn't trigger it. Alan Stern
