On Thu, May 23, 2019 at 9:05 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
>
> No other register/unregister kernel API attempts to provide this kind of
> protection as it is inherently racy, so just drop it.
>
> Callers should provide their own protection, it appears nouveau already
> does, but just in case drop a debugging POISON.
>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> ---
> mm/hmm.c | 9 ++-------
> 1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 46872306f922bb..6c3b7398672c29 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -286,18 +286,13 @@ EXPORT_SYMBOL(hmm_mirror_register);
> */
> void hmm_mirror_unregister(struct hmm_mirror *mirror)
> {
> - struct hmm *hmm = READ_ONCE(mirror->hmm);
> -
> - if (hmm == NULL)
> - return;
> + struct hmm *hmm = mirror->hmm;
How about remove struct hmm *hmm and replace the code like below -
down_write(&mirror->hmm->mirrors_sem);
list_del_init(&mirror->list);
up_write(&mirror->hmm->mirrors_sem);
hmm_put(hmm);
memset(&mirror->hmm, POISON_INUSE, sizeof(mirror->hmm));
Similar to hmm_mirror_register().
> down_write(&hmm->mirrors_sem);
> list_del_init(&mirror->list);
> - /* To protect us against double unregister ... */
> - mirror->hmm = NULL;
> up_write(&hmm->mirrors_sem);
> -
> hmm_put(hmm);
> + memset(&mirror->hmm, POISON_INUSE, sizeof(mirror->hmm));
> }
> EXPORT_SYMBOL(hmm_mirror_unregister);
>
> --
> 2.21.0
>
