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2025 at 05:04:09PM +0100, Vlastimil Babka wrote: > > > On 11/26/25 4:20 PM, Lorenzo Stoakes wrote: > > On Wed, Nov 26, 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() > > I think it's refcount is NOT zero to continue with __vma_enter_locked(). Yup sorry, misread the ! clearly... Which makes a lot more sense when it comes to picking up spurious reader refcount increases :) > > > (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) > > That shouldn't be possible, yeah. But per above, it's actually not zero. Yup so this just makes it more likely to happen... > > > (back to vma_mark_attached() -> __vma_enter_locked()) > > Back to _attached()? but it's _detached() above? You mean _detached() > right? Just to be sure Yup, I typed this all a bit too quick... > > > -> refcount_add_not_zero() returns true > > Ack. > > > [ process gets fatal signal ] > > > > -> rcuwait_wait_event() errors out > > -> oopsies need to do something, maybe [VM_]WARN_ON() not right? > > AFAICS from vma_mark_detached() we use the TASK_UNINTERRUPTIBLE variant > so this path can't error due to the fatal signal. Right good point. I hate that we make this so 'gosh darned' implicit. We are now assuming that: 1. the only way that RCU wait can fail is due to pending fatal signal 2. and that we're fine here because it's uninterruptible. I mean very doubtful we'll ever change that but it's still gross. And as Willy says we're paving the road with good intent^Wlandmines. > > > Correct me if the above is wrong. Yeah I was wrong thankfully :) The TASK_UNINTERRUPTIBLE saves us, but it's all still a bit ugh. > > > > 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? > > Yeah I guess it's for the best to keep vma_mark_detached() use the > TASK_UNINTERRUPTIBLE variant, maybe document why. Aborting the detaching > would be counter productive. > > > I actually wonder if a WARN_ON() is warranted to see if this even ever > > happens... > > Not for this path, but for vma_start_write_killable -> __vma_start_write > -> __vma_enter_locked(... TASK_KILLABLE). I think it still can't Well if it's impossible for TASK_UNINTERRUPTIBLE no harm in adding it right? Can add a comment. > trigger, but since we need to check result of the > refcount_sub_and_test() anyway, we might as well WARN_ON it. Probably it can't no. > > > OK just going to reattach... my head which just exploded from the above :P > > > > Cheers, Lorenzo > > Thanks, Lorenzo