Re: [PATCH v14 10/25] nfsd: add nfsd_serv_try_get and nfsd_serv_put

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2024-08-28 at 21:04 -0400, Mike Snitzer wrote:
> Introduce nfsd_serv_try_get and nfsd_serv_put and update the nfsd code
> to prevent nfsd_destroy_serv from destroying nn->nfsd_serv until any
> caller of nfsd_serv_try_get releases their reference using nfsd_serv_put.
> 
> A percpu_ref is used to implement the interlock between
> nfsd_destroy_serv and any caller of nfsd_serv_try_get.
> 
> This interlock is needed to properly wait for the completion of client
> initiated localio calls to nfsd (that are _not_ in the context of nfsd).
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
>  fs/nfsd/netns.h  |  8 +++++++-
>  fs/nfsd/nfssvc.c | 39 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 46 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
> index 238fc4e56e53..e2d953f21dde 100644
> --- a/fs/nfsd/netns.h
> +++ b/fs/nfsd/netns.h
> @@ -13,6 +13,7 @@
>  #include <linux/filelock.h>
>  #include <linux/nfs4.h>
>  #include <linux/percpu_counter.h>
> +#include <linux/percpu-refcount.h>
>  #include <linux/siphash.h>
>  #include <linux/sunrpc/stats.h>
>  
> @@ -139,7 +140,9 @@ struct nfsd_net {
>  
>  	struct svc_info nfsd_info;
>  #define nfsd_serv nfsd_info.serv
> -
> +	struct percpu_ref nfsd_serv_ref;
> +	struct completion nfsd_serv_confirm_done;
> +	struct completion nfsd_serv_free_done;
>  
>  	/*
>  	 * clientid and stateid data for construction of net unique COPY
> @@ -221,6 +224,9 @@ struct nfsd_net {
>  extern bool nfsd_support_version(int vers);
>  extern unsigned int nfsd_net_id;
>  
> +bool nfsd_serv_try_get(struct nfsd_net *nn);
> +void nfsd_serv_put(struct nfsd_net *nn);
> +
>  void nfsd_copy_write_verifier(__be32 verf[2], struct nfsd_net *nn);
>  void nfsd_reset_write_verifier(struct nfsd_net *nn);
>  #endif /* __NFSD_NETNS_H__ */
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index defc430f912f..e43d440f9f0a 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -193,6 +193,30 @@ int nfsd_minorversion(struct nfsd_net *nn, u32 minorversion, enum vers_op change
>  	return 0;
>  }
>  
> +bool nfsd_serv_try_get(struct nfsd_net *nn)
> +{
> +	return percpu_ref_tryget_live(&nn->nfsd_serv_ref);
> +}
> +
> +void nfsd_serv_put(struct nfsd_net *nn)
> +{
> +	percpu_ref_put(&nn->nfsd_serv_ref);
> +}
> +
> +static void nfsd_serv_done(struct percpu_ref *ref)
> +{
> +	struct nfsd_net *nn = container_of(ref, struct nfsd_net, nfsd_serv_ref);
> +
> +	complete(&nn->nfsd_serv_confirm_done);
> +}
> +
> +static void nfsd_serv_free(struct percpu_ref *ref)
> +{
> +	struct nfsd_net *nn = container_of(ref, struct nfsd_net, nfsd_serv_ref);
> +
> +	complete(&nn->nfsd_serv_free_done);
> +}
> +
>  /*
>   * Maximum number of nfsd processes
>   */
> @@ -392,6 +416,7 @@ static void nfsd_shutdown_net(struct net *net)
>  		lockd_down(net);
>  		nn->lockd_up = false;
>  	}
> +	percpu_ref_exit(&nn->nfsd_serv_ref);
>  	nn->nfsd_net_up = false;
>  	nfsd_shutdown_generic();
>  }
> @@ -471,6 +496,13 @@ void nfsd_destroy_serv(struct net *net)
>  	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
>  	struct svc_serv *serv = nn->nfsd_serv;
>  
> +	lockdep_assert_held(&nfsd_mutex);
> +
> +	percpu_ref_kill_and_confirm(&nn->nfsd_serv_ref, nfsd_serv_done);
> +	wait_for_completion(&nn->nfsd_serv_confirm_done);
> +	wait_for_completion(&nn->nfsd_serv_free_done);
> +	/* percpu_ref_exit is called in nfsd_shutdown_net */
> +
>  	spin_lock(&nfsd_notifier_lock);
>  	nn->nfsd_serv = NULL;
>  	spin_unlock(&nfsd_notifier_lock);
> @@ -595,6 +627,13 @@ int nfsd_create_serv(struct net *net)
>  	if (nn->nfsd_serv)
>  		return 0;
>  
> +	error = percpu_ref_init(&nn->nfsd_serv_ref, nfsd_serv_free,
> +				0, GFP_KERNEL);
> +	if (error)
> +		return error;
> +	init_completion(&nn->nfsd_serv_free_done);
> +	init_completion(&nn->nfsd_serv_confirm_done);
> +
>  	if (nfsd_max_blksize == 0)
>  		nfsd_max_blksize = nfsd_get_default_max_blksize();
>  	nfsd_reset_versions(nn);

A little hard to review this one at this point in the series, as there
are no callers of get/put yet, but the concept seems reasonable.

Reviewed-by: Jeff Layton <jlayton@kernel.org>