On Tue, Jun 22, 2021 at 06:51:13PM +0200, Greg KH wrote: > On Tue, Jun 22, 2021 at 09:40:27AM -0700, Luis Chamberlain wrote: > > On Tue, Jun 22, 2021 at 06:27:52PM +0200, Greg KH wrote: > > > On Tue, Jun 22, 2021 at 08:27:13AM -0700, Luis Chamberlain wrote: > > > > On Tue, Jun 22, 2021 at 09:41:23AM +0200, Greg KH wrote: > > > > > On Mon, Jun 21, 2021 at 04:36:34PM -0700, Luis Chamberlain wrote: > > > > > > + ssize_t __ret; \ > > > > > > + if (!try_module_get(THIS_MODULE)) \ > > > > > > > > > > try_module_get(THIS_MODULE) is always racy and probably does not do what > > > > > you want it to do. You always want to get/put module references from > > > > > code that is NOT the code calling these functions. > > > > > > > > In this case, we want it to trump module removal if it succeeds. That's all. > > > > > > True, but either you stop the race, or you do not right? If you are so > > > invested in your load/unload test, this should show up with this code > > > eventually as well. > > > > I still do not see how the race is possible give the goal to prevent > > module removal if a sysfs file is being used. If rmmod is taking > > place, this simply will bail out. > > > > > > > > + return -ENODEV; \ > > > > > > + __ret = _name ## _store(dev, attr, buf, len); \ > > > > > > + module_put(THIS_MODULE); \ > > > > > > > > > > This too is going to be racy. > > > > > > > > > > While fun to poke at, I still think this is pointless. > > > > > > > > If you have a better idea, which does not "DOS" module removal, please > > > > let me know! > > > > > > I have yet to understand why you think that the load/unload in a loop is > > > a valid use case. > > > > That is dependent upon the intrastructure tests built for a driver. > > > > In the case of fstests and blktests we have drivers which *always* get > > removed and loaded on each test. Take for instance scsi_debug, which > > creates / destroys virtual devices on the per test. Likewise, to build > > confidence that failure rate is as close as possible to 0, one must run > > a test as many times as possible in a loop. And, to build confidence in > > a test, in some situations one ends up running modprobe / rmmod in a > > loop. > > > > In this case a customer does have a complex system of tests, and by looking > > at the crash logs I managed to simplify the way to reproduce it using > > simple shell scripts. > > And is _this_ change needed even with the changes in patch 1/3? Oh absolutely. This patch is needed 100%. Without it, it is actually pretty trivial to deadlock as noted in my instructions on how to reproduce. > I think that commit fixes your issues given that you will not unload the > module until after the sysfs devices are removed from the system. Have > you tried that alone with your test? I have tried that, and it does not resolve the deadlock. It was *why* I have been insisting that this is a real issue, and why I decided to instead try to implement something generic after I was hinted by livepatch folks that they also had observed a similar deadlock, and so that a generic solution would be appreciated by them. Luis
