Re: [PATCH 03/14] mm/hmm: HMM should have a callback before MM is destroyed v2

[John Hubbard]

On 03/16/2018 12:14 PM, jglisse@xxxxxxxxxx wrote:
> From: Ralph Campbell <rcampbell@xxxxxxxxxx>
> 

<snip>

> +static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
> +{
> +	struct hmm *hmm = mm->hmm;
> +	struct hmm_mirror *mirror;
> +	struct hmm_mirror *mirror_next;
> +
> +	down_write(&hmm->mirrors_sem);
> +	list_for_each_entry_safe(mirror, mirror_next, &hmm->mirrors, list) {
> +		list_del_init(&mirror->list);
> +		if (mirror->ops->release)
> +			mirror->ops->release(mirror);
> +	}
> +	up_write(&hmm->mirrors_sem);
> +}
> +

OK, as for actual code review:

This part of the locking looks good. However, I think it can race against
hmm_mirror_register(), because hmm_mirror_register() will just add a new 
mirror regardless.

So:

thread 1                                      thread 2
--------------                                -----------------
hmm_release                                   hmm_mirror_register 
    down_write(&hmm->mirrors_sem);                <blocked: waiting for sem>
        // deletes all list items
    up_write
                                                  unblocked: adds new mirror
                                              

...so I think we need a way to back out of any pending hmm_mirror_register()
calls, as part of the .release steps, right? It seems hard for the device driver,
which could be inside of hmm_mirror_register(), to handle that. Especially considering
that right now, hmm_mirror_register() will return success in this case--so
there is no indication that anything is wrong.

Maybe hmm_mirror_register() could return an error (and not add to the mirror list),
in such a situation, how's that sound?

thanks,
-- 
John Hubbard
NVIDIA