On Thu, Sep 01, 2022 at 01:16:54PM -0600, Logan Gunthorpe wrote: > > > On 2022-09-01 12:36, Greg Kroah-Hartman wrote: > > On Thu, Sep 01, 2022 at 12:14:25PM -0600, Logan Gunthorpe wrote: > >> Well we haven't plugged in a remove call into p2pdma, that would be more > >> work and more interfaces touching the PCI code. Note: this code isn't a > >> driver but a set of PCI helpers available to other PCI drivers. > >> Everything that's setup is using the devm interfaces and gets torn down > >> with the same. So I don't really see the benefit of making the change > >> you propose. > > > > The issue is the classic one with the devm helpers. They do not lend > > themselves to resource management issues that require ordering or other > > sort of dependencies. Please do not use them here, just put in a remove > > callback as you eventually will need it anyway, as you have a strong > > requirement for what gets freed when, and the devm api does not provide > > for that well. > > This surprises me. Can you elaborate on this classic issue? There's long threads about it on the ksummit discuss mailing list and other places. > I've definitely seen uses of devm that expect the calls will be torn > down in reverse order they are added. Sorry, I didn't mean to imply the ordering of the devm code is incorrect, that's fine. It's when you have things in the devm "chain" that need to be freed in a different order that stuff gets messy. Like irqs and clocks and other types of resources that have "actions" associated with them. > The existing p2pdma code will > certainly fail quite significantly if a devm_kzalloc() releases its > memory before the devm_memmap_pages() cleans up. There's also already an > action that is used to cleanup before the last devm_kzalloc() call > happens. If ordering is not guaranteed, then devm seems fairly broken > and unusable and I'd have to drop all uses from this code and go back to > the error prone method. Also what's the point of > devm_add_action_or_reset() if it doesn't guarantee the ordering or the > release? I have never used devm_add_action_or_reset() so I can't say why it is there. I am just pointing out that manually messing with a sysfs group from a driver is a huge flag that something is wrong. A driver should almost never be touching a raw kobject or calling any sysfs_* call if all is normal, which is why I questioned this. > But if it's that important I can make the change to these patches for v10. Try it the way I suggest, with a remove() callback, and see if that looks simpler and easier to follow and maintain over time. thanks, greg k-h
