On Tue, Jan 17, 2023 at 10:03 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> +locking maintainers
Thanks! I'll CC the locking maintainers in the next posting.
>
> On Mon, Jan 9, 2023 at 9:54 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > Introduce a per-VMA rw_semaphore to be used during page fault handling
> > instead of mmap_lock. Because there are cases when multiple VMAs need
> > to be exclusively locked during VMA tree modifications, instead of the
> > usual lock/unlock patter we mark a VMA as locked by taking per-VMA lock
> > exclusively and setting vma->lock_seq to the current mm->lock_seq. When
> > mmap_write_lock holder is done with all modifications and drops mmap_lock,
> > it will increment mm->lock_seq, effectively unlocking all VMAs marked as
> > locked.
> [...]
> > +static inline void vma_read_unlock(struct vm_area_struct *vma)
> > +{
> > + up_read(&vma->lock);
> > +}
>
> One thing that might be gnarly here is that I think you might not be
> allowed to use up_read() to fully release ownership of an object -
> from what I remember, I think that up_read() (unlike something like
> spin_unlock()) can access the lock object after it's already been
> acquired by someone else. So if you want to protect against concurrent
> deletion, this might have to be something like:
>
> rcu_read_lock(); /* keeps vma alive */
> up_read(&vma->lock);
> rcu_read_unlock();
But for deleting VMA one would need to write-lock the vma->lock first,
which I assume can't happen until this up_read() is complete. Is that
assumption wrong?
>
> But I'm not entirely sure about that, the locking folks might know better.
>
> Also, it might not matter given that the rw_semaphore part is removed
> in the current patch 41/41 anyway...
This does matter because Michal suggested dropping that last 41/41
patch for now.
