Re: [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v2

[John Hubbard]

On 03/21/2018 11:16 AM, jglisse@xxxxxxxxxx wrote:
> From: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> 
> This code was lost in translation at one point. This properly call
> mmu_notifier_unregister_no_release() once last user is gone. This
> fix the zombie mm_struct as without this patch we do not drop the
> refcount we have on it.
> 
> Changed since v1:
>   - close race window between a last mirror unregistering and a new
>     mirror registering, which could have lead to use after free()
>     kind of bug
> 
> Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Cc: Evgeny Baskakov <ebaskakov@xxxxxxxxxx>
> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Cc: Mark Hairgrove <mhairgrove@xxxxxxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
>  mm/hmm.c | 35 +++++++++++++++++++++++++++++++++--
>  1 file changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 6088fa6ed137..f75aa8df6e97 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -222,13 +222,24 @@ int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm)
>  	if (!mm || !mirror || !mirror->ops)
>  		return -EINVAL;
>  
> +again:
>  	mirror->hmm = hmm_register(mm);
>  	if (!mirror->hmm)
>  		return -ENOMEM;
>  
>  	down_write(&mirror->hmm->mirrors_sem);
> -	list_add(&mirror->list, &mirror->hmm->mirrors);
> -	up_write(&mirror->hmm->mirrors_sem);
> +	if (mirror->hmm->mm == NULL) {
> +		/*
> +		 * A racing hmm_mirror_unregister() is about to destroy the hmm
> +		 * struct. Try again to allocate a new one.
> +		 */
> +		up_write(&mirror->hmm->mirrors_sem);
> +		mirror->hmm = NULL;

This is being set outside of locks, so now there is another race with
another hmm_mirror_register...

I'll take a moment and draft up what I have in mind here, which is a more
symmetrical locking scheme for these routines.

thanks,
-- 
John Hubbard
NVIDIA