On Tue, 05 Dec 2023, Christian Brauner wrote: > On Mon, Dec 04, 2023 at 03:09:44PM -0700, Jens Axboe wrote: > > On 12/4/23 2:02 PM, NeilBrown wrote: > > > It isn't clear to me what _GPL is appropriate, but maybe the rules > > > changed since last I looked..... are there rules? > > > > > > My reasoning was that the call is effectively part of the user-space > > > ABI. A user-space process can call this trivially by invoking any > > > system call. The user-space ABI is explicitly a boundary which the GPL > > > does not cross. So it doesn't seem appropriate to prevent non-GPL > > > kernel code from doing something that non-GPL user-space code can > > > trivially do. > > > > By that reasoning, basically everything in the kernel should be non-GPL > > marked. And while task_work can get used by the application, it happens > > only indirectly or implicitly. So I don't think this reasoning is sound > > at all, it's not an exported ABI or API by itself. > > > > For me, the more core of an export it is, the stronger the reason it > > should be GPL. FWIW, I don't think exporting task_work functionality is > > a good idea in the first place, but if there's a strong reason to do so, > > Yeah, I'm not too fond of that part as well. I don't think we want to > give modules the ability to mess with task work. This is just asking for > trouble. > Ok, maybe we need to reframe the problem then. Currently fput(), and hence filp_close(), take control away from kernel threads in that they cannot be sure that a "close" has actually completed. This is already a problem for nfsd. When renaming a file, nfsd needs to ensure any cached "open" that it has on the file is closed (else when re-exporting an NFS filesystem it can result in a silly-rename). nfsd currently handles this case by calling flush_delayed_fput(). I suspect you are no more happy about exporting that than you are about exporting task_work_run(), but this solution isn't actually 100% reliable. If some other thread calls flush_delayed_fput() between nfsd calling filp_close() and that same nfsd calling flush_delayed_fput(), then the second flush can return before the first flush (in the other thread) completes all the work it took on. What we really need - both for handling renames and for avoiding possible memory exhaustion - is for nfsd to be able to reliably wait for any fput() that it initiated to complete. How would you like the VFS to provide that service? Thanks, NeilBrown
