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2025 at 03:05:44PM +0000, Matthew Wilcox wrote: > On Wed, Nov 26, 2025 at 03:36:46PM +0100, Vlastimil Babka wrote: > > On 11/26/25 5:28 AM, Suren Baghdasaryan wrote: > > >> Suren, Liam, Vlastimil, Lorenzo ... none of you spotted this bug. > > > > > > Doh! This is embarassing... > > > > Hand-rolled synchronization primitives are wonderful, aren't they? > > That's why I liked the original approach of just using rwsems. I > mst admit to having not paid attention to this recently so I don't > know what motivated the change. > > > > Wait, why do we consider this as a successful acquisition? The > > > vm_refcnt is 0, so this is similar situation to an earlier: > > > > > > if (!refcount_add_not_zero(VMA_LOCK_OFFSET, &vma->vm_refcnt)) > > > return 0; > > > > But this means "vma is not attached" not "we failed to lock it". > > > > > IOW, the vma is not referenced, so we failed to lock it. I think the > > > fix should be: > > > > > > if (err) { > > > + if (refcount_sub_and_test(VMA_LOCK_OFFSET, &vma->vm_refcnt)) { > > > + /* Oh cobblers. While we got a fatal signal, we > > > + * raced with the last user. VMA is not referenced, > > > + * fail to lock it. > > > + */ > > > + err = 0; > > > > Returning 0 in this situation therefore wouldn't be correct. > > > > AFAIU since we started with attached vma above, it's not possible that > > the refcount_sub_and_test here will drop the refcnt to zero. We could > > just WARN_ON_ONCE() on the result (in a way to make also the > > __must_check happy) and then can return err below. > > But how do we know that we started with an attached VMA? Maybe the VMA > was in the process of being detached and still has readers? So we're talking about: vma_mark_deteched() -> refcount_dec_and_test() [ ref count is zero ] -> __vma_enter_locked() (meanwhile...) -> reader attempts to read -> optimistic check doesn't successfully find write locked VMA -> __refcount_inc_not_zero_limited_acqure() somehow doesn't notice 0 refcount and increments (??? how) (back to vma_mark_attached() -> __vma_enter_locked()) -> refcount_add_not_zero() returns true [ process gets fatal signal ] -> rcuwait_wait_event() errors out -> oopsies need to do something, maybe [VM_]WARN_ON() not right? Correct me if the above is wrong. I mean is any of this actually possible...? Seems dubious. But I guess right now we assume it _is_ possible. What a mess! (Again I wonder why we made our lives so difficult here) Anyway even if we are midway through a detach, the detach is ostensibly waiting for the readers to go away, and our reader is about to go away anyway, but the process has a fatal signal so do we even care? I actually wonder if a WARN_ON() is warranted to see if this even ever happens... OK just going to reattach... my head which just exploded from the above :P Cheers, Lorenzo