On Thu, Jul 22, 2021 at 02:31:37PM -0700, Luis Chamberlain wrote: > On Wed, Jul 21, 2021 at 01:30:29PM +0200, Greg KH wrote: > > On Thu, Jul 01, 2021 at 03:48:16PM -0700, Luis Chamberlain wrote: > > > On Fri, Jun 25, 2021 at 02:56:03PM -0700, Luis Chamberlain wrote: > > > > On Thu, Jun 24, 2021 at 01:09:03PM +0200, Greg KH wrote: > > > > > thanks for making this change and sticking with it! > > > > > > > > > > Oh, and with this change, does your modprobe/rmmod crazy test now work? > > > > > > > > It does but I wrote a test_syfs driver and I believe I see an issue with > > > > this. I'll debug a bit more and see what it was, and I'll then also use > > > > the driver to demo the issue more clearly, and then verification can be > > > > an easy selftest test. > > > > > > OK my conclusion based on a new selftest driver I wrote is we can drop > > > this patch safely. The selftest will cover this corner case well now. > > > > > > In short: the kernfs active reference will ensure the store operation > > > still exists. The kernfs mutex is not enough, but if the driver removes > > > the operation prior to getting the active reference, the write will just > > > fail. The deferencing inside of the sysfs operation is abstract to > > > kernfs, and while kernfs can't do anything to prevent a driver from > > > doing something stupid, it at least can ensure an open file ensure the > > > op is not removed until the operation completes. > > > > Ok, so all is good? > > It would seem to be the case. > > > Then why is your zram test code blowing up so badly? > > I checked the logs for the backtrace where the crash did happen > and we did see clear evidence of the race we feared here. The *first* > bug that happened was the CPU hotplug race: > > [132004.787099] Error: Removing state 61 which has instances left. > [132004.787124] WARNING: CPU: 17 PID: 9307 at ../kernel/cpu.c:1879 __cpuhp_remove_state_cpuslocked+0x1c4/0x1d0 I do not understand what this issue is, is it fixed? Why is a cpu being hot unplugged at the same time a zram? thanks, greg k-h
