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McKenney wrote: > On Thu, Nov 13, 2025 at 12:04:19AM +0000, Matthew Wilcox wrote: > > On Wed, Nov 12, 2025 at 03:06:38PM +0000, Lorenzo Stoakes wrote: > > > > Any time the rcu read lock is dropped, the maple state must be > > > > invalidated. Resetting the address and state to MA_START is the safest > > > > course of action, which will result in the next operation starting from > > > > the top of the tree. > > > > > > Since we all missed it I do wonder if we need some super clear comment > > > saying 'hey if you drop + re-acquire RCU lock you MUST revalidate mas state > > > by doing 'blah'. > > > > I mean, this really isn't an RCU thing. This is also bad: > > > > spin_lock(a); > > p = *q; > > spin_unlock(a); > > spin_lock(a); > > b = *p; > > > > p could have been freed while you didn't hold lock a. Detecting this > > kind of thing needs compiler assistence (ie Rust) to let you know that > > you don't have the right to do that any more. > > While in no way denigrating Rust's compile-time detection of this sort > of thing, use of KASAN combined with CONFIG_RCU_STRICT_GRACE_PERIOD=y > (which restricts you to four CPUs) can sometimes help. > > > > I think one source of confusion for me with maple tree operations is - what > > > to do if we are in a position where some kind of reset is needed? > > > > > > So even if I'd realised 'aha we need to reset this' it wouldn't be obvious > > > to me that we ought to set to the address. > > > > I think that's a separate problem. > > > > > > +++ b/mm/mmap_lock.c > > > > @@ -257,6 +257,7 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, > > > > if (PTR_ERR(vma) == -EAGAIN) { > > > > count_vm_vma_lock_event(VMA_LOCK_MISS); > > > > /* The area was replaced with another one */ > > > > + mas_set(&mas, address); > > > > > > I wonder if we could detect that the RCU lock was released (+ reacquired) in > > > mas_walk() in a debug mode, like CONFIG_VM_DEBUG_MAPLE_TREE? > > > > Dropping and reacquiring the RCU read lock should have been a big red > > flag. I didn't have time to review the patches, but if I had, I would > > have suggested passing the mas down to the routine that drops the rcu > > read lock so it can be invalidated before dropping the readlock. > > There has been some academic efforts to check for RCU-protected pointers > leaking from one RCU read-side critical section to another, but nothing > useful has come from this. :-/ Ugh a pity. I was hoping we could do (in debug mode only obv) something absolutely roughly like: On init: mas->rcu_critical_section = rcu_get_critical_section_blah(); ... On walk: VM_WARN_ON(rcu_critical_section_blah() != mas->rcu_critical_section); But sounds like that isn't feasible. I always like the idea of us having debug stuff that helps highlight dumb mistakes very quickly, no matter how silly they might be :) > > But rcu_pointer_handoff() and unrcu_pointer() are intended not only for > documentation, but also to suppress the inevitable false positives should > anyone figure out how to detect leaking of RCU-protected pointers. > > Thanx, Paul Cheers, Lorenzo