Re: [PATCH v6 15/18] nfsd: use SRCU to dereference nn->nfsd_serv

NeilBrown
2024-06-21T06:35:21Z

On Thu, 20 Jun 2024, Mike Snitzer wrote:
> Introduce nfsd_serv_get, nfsd_serv_put and nfsd_serv_sync and update
> the nfsd code to prevent nfsd_destroy_serv from destroying
> nn->nfsd_serv until all nfsd code is done with it (particularly the
> localio code that doesn't run in the context of nfsd's svc threads,
> nor does it take the nfsd_mutex).
>
> Commit 83d5e5b0af90 ("dm: optimize use SRCU and RCU") provided a
> familiar well-worn pattern for how implement.
>
> Suggested-by: NeilBrown <neilb@suse.de>
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
> fs/nfsd/filecache.c | 13 ++++++++---
> fs/nfsd/netns.h | 14 ++++++++++--
> fs/nfsd/nfs4state.c | 25 ++++++++++++++-------
> fs/nfsd/nfsctl.c | 7 ++++--
> fs/nfsd/nfssvc.c | 54 ++++++++++++++++++++++++++++++++++++---------
> 5 files changed, 88 insertions(+), 25 deletions(-)
>
> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> index 99631fa56662..474b3a3af3fb 100644
> --- a/fs/nfsd/filecache.c
> +++ b/fs/nfsd/filecache.c
> @@ -413,12 +413,15 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose)
> struct nfsd_file *nf = list_first_entry(dispose,
> struct nfsd_file, nf_lru);
> struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id);
> + int srcu_idx;
> + struct svc_serv *serv = nfsd_serv_get(nn, &srcu_idx);
> struct nfsd_fcache_disposal *l = nn->fcache_disposal;
>
> spin_lock(&l->lock);
> list_move_tail(&nf->nf_lru, &l->freeme);
> spin_unlock(&l->lock);
> - svc_wake_up(nn->nfsd_serv);
> + svc_wake_up(serv);
> + nfsd_serv_put(nn, srcu_idx);
> }
> }
>
> @@ -443,11 +446,15 @@ void nfsd_file_net_dispose(struct nfsd_net *nn)
> for (i = 0; i < 8 && !list_empty(&l->freeme); i++)
> list_move(l->freeme.next, &dispose);
> spin_unlock(&l->lock);
> - if (!list_empty(&l->freeme))
> + if (!list_empty(&l->freeme)) {
> + int srcu_idx;
> + struct svc_serv *serv = nfsd_serv_get(nn, &srcu_idx);
> /* Wake up another thread to share the work
> * *before* doing any actual disposing.
> */
> - svc_wake_up(nn->nfsd_serv);
> + svc_wake_up(serv);
> + nfsd_serv_put(nn, srcu_idx);
> + }
> nfsd_file_dispose_list(&dispose);
> }
> }
> diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
> index 0c5a1d97e4ac..92d0d0883f17 100644
> --- a/fs/nfsd/netns.h
> +++ b/fs/nfsd/netns.h
> @@ -139,8 +139,14 @@ struct nfsd_net {
> u32 clverifier_counter;
>
> struct svc_info nfsd_info;
> -#define nfsd_serv nfsd_info.serv
> -
> + /*
> + * The current 'nfsd_serv' at nfsd_info.serv. Using 'void' rather than
> + * 'struct svc_serv' to guard against new code dereferencing nfsd_serv
> + * without using proper synchronization.
> + * Use nfsd_serv_get() or take nfsd_mutex to dereference.
> + */
> + void __rcu *nfsd_serv;
> + struct srcu_struct nfsd_serv_srcu;
>

srcu_struct is not tiny. I think it would make sense to use a global
struct for all net namespaces.

Most users do seem to be embed them in some other structure - but not
all.... kfd_processes_srcu stm_source_srcu

Thanks,
NeilBrown