On Wed, Feb 04, 2026 at 10:03:33PM -0500, Waiman Long wrote: > Now I realize that there is indeed a deadlock problem. Scrap that. Now I > have a simpler idea that shouldn't have this type of deadlock problem. So > what do you think about the sample code below?
That it's rather bizarre, TBH. Basically, you are allowing to park a number of (identical) references in there instead of dropping them, with your 'xrefs' being the count of skipped drops. get_share either clones a reference or uses up one of those skipped drops; put_share parks the reference if possible. And set discards everything not used up... It could be made to work, but... ouch. It looks like a special-cased variant of something fairly generic, with really confusing calling conventions. Let me poke around and see if we have any other candidates for something similar; if nothing else, current->fs->root is interesting and not just for audit pathologies... Note, BTW, that there's chroot_fs_refs() to deal with, along with free_fs_struct() (at least). This stuff is encapsulated in fs/fs_struct.c and include/linux/fs_struct.h... Oh, hell. "fs: inline current_umask() and move it to fs_struct.h" had been a bad idea; I'd missed it when it had been posted, but... exposing the damn thing all over the place *and* bringing sched.h into it? Microoptimizations are fine, and this one might have a measurable effect, but it grows the subset of the kernel we need to audit when we deal with changing the damn object by at least a couple of orders of magnitude ;-/ Sigh... Mateusz: *is* there a measurable effect? Because if there isn't, I'm very tempted to simply revert that thing. "Churn of adding fs_struct.h as needed" is not the problem - try "exposing the object internals to far larger subset of the kernel". We had interesting bugs with weird shit deciding to poke in there, locking and refcounting be damned. Encapsulation is really called for in some cases... And yes, I ought to have caught that one back in November and asked _then_; mea culpa.
