Re: [PATCH v2 3/5] ceph: add wait_list_lock for wait-list serialization

[Viacheslav Dubeyko]

On Wed, 2026-07-15 at 11:58 +0800, Xiubo Li via B4 Relay wrote:
> From: Xiubo Li <xiubo.li@xxxxxxxxx>
> 
> Add a dedicated spinlock to protect the waiting_for_map and
> s_waiting lists, which are currently serialized by mdsc->mutex.
> This is a preparatory step for removing mdsc->mutex from the
> kick_requests() path: xa_for_each() is internally locked and the
> per-request filter checks are lockless, so only list_del_init()
> needs external protection.
> 
> Signed-off-by: Xiubo Li <xiubo.li@xxxxxxxxx>
> ---
>  fs/ceph/mds_client.c | 18 +++++++++++++++++-
>  fs/ceph/mds_client.h |  3 +++
>  2 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> index 239ed34886f9..a250938f32d3 100644
> --- a/fs/ceph/mds_client.c
> +++ b/fs/ceph/mds_client.c
> @@ -3669,7 +3669,9 @@ static void __do_request(struct ceph_mds_client
> *mdsc,
>  			doutc(cl, "no mdsmap, waiting for map\n");
>  			trace_ceph_mdsc_suspend_request(mdsc,
> session, req,
>  							ceph_mdsc_su
> spend_reason_no_mdsmap);
> +			spin_lock(&mdsc->wait_list_lock);
>  			list_add(&req->r_wait, &mdsc-
> >waiting_for_map);
> +			spin_unlock(&mdsc->wait_list_lock);
>  			return;
>  		}
>  		if (!(mdsc->fsc->mount_options->flags &
> @@ -3692,7 +3694,9 @@ static void __do_request(struct ceph_mds_client
> *mdsc,
>  		doutc(cl, "no mds or not active, waiting for
> map\n");
>  		trace_ceph_mdsc_suspend_request(mdsc, session, req,
>  						ceph_mdsc_suspend_re
> ason_no_active_mds);
> +		spin_lock(&mdsc->wait_list_lock);
>  		list_add(&req->r_wait, &mdsc->waiting_for_map);
> +		spin_unlock(&mdsc->wait_list_lock);
>  		return;
>  	}
>  
> @@ -3740,9 +3744,12 @@ static void __do_request(struct
> ceph_mds_client *mdsc,
>  			if (ceph_test_mount_opt(mdsc->fsc,
> CLEANRECOVER)) {
>  				trace_ceph_mdsc_suspend_request(mdsc
> , session, req,
>  								ceph
> _mdsc_suspend_reason_rejected);
> +				spin_lock(&mdsc->wait_list_lock);
>  				list_add(&req->r_wait, &mdsc-
> >waiting_for_map);
> -			} else
> +				spin_unlock(&mdsc->wait_list_lock);
> +			} else {
>  				err = -EACCES;
> +			}

Do we really need to add the curly brackets here?

Thanks,
Slava.

>  			goto out_session;
>  		}
>  
> @@ -3757,7 +3764,9 @@ static void __do_request(struct ceph_mds_client
> *mdsc,
>  		}
>  		trace_ceph_mdsc_suspend_request(mdsc, session, req,
>  						ceph_mdsc_suspend_re
> ason_session);
> +		spin_lock(&mdsc->wait_list_lock);
>  		list_add(&req->r_wait, &session->s_waiting);
> +		spin_unlock(&mdsc->wait_list_lock);
>  		goto out_session;
>  	}
>  
> @@ -3850,7 +3859,9 @@ static void __wake_requests(struct
> ceph_mds_client *mdsc,
>  	struct ceph_mds_request *req;
>  	LIST_HEAD(tmp_list);
>  
> +	spin_lock(&mdsc->wait_list_lock);
>  	list_splice_init(head, &tmp_list);
> +	spin_unlock(&mdsc->wait_list_lock);
>  
>  	while (!list_empty(&tmp_list)) {
>  		req = list_entry(tmp_list.next,
> @@ -3892,7 +3903,9 @@ static void kick_requests(struct
> ceph_mds_client *mdsc, int mds)
>  		if (req->r_session &&
>  		    req->r_session->s_mds == mds) {
>  			doutc(cl, " kicking tid %llu\n", req-
> >r_tid);
> +			spin_lock(&mdsc->wait_list_lock);
>  			list_del_init(&req->r_wait);
> +			spin_unlock(&mdsc->wait_list_lock);
>  			trace_ceph_mdsc_resume_request(mdsc, req);
>  			__do_request(mdsc, req);
>  		}
> @@ -6359,6 +6372,7 @@ int ceph_mdsc_init(struct ceph_fs_client *fsc)
>  	mdsc->snap_realms = RB_ROOT;
>  	INIT_LIST_HEAD(&mdsc->snap_empty);
>  	spin_lock_init(&mdsc->snap_empty_lock);
> +	spin_lock_init(&mdsc->wait_list_lock);
>  #if BITS_PER_LONG == 64
>  	xa_init(&mdsc->request_tree);
>  #else
> @@ -6445,7 +6459,9 @@ static void wait_requests(struct
> ceph_mds_client *mdsc)
>  		mutex_lock(&mdsc->mutex);
>  		while ((req = __get_oldest_req(mdsc))) {
>  			doutc(cl, "timed out on tid %llu\n", req-
> >r_tid);
> +			spin_lock(&mdsc->wait_list_lock);
>  			list_del_init(&req->r_wait);
> +			spin_unlock(&mdsc->wait_list_lock);
>  			__unregister_request(mdsc, req);
>  		}
>  	}
> diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
> index c2de551607e3..0a9f89498604 100644
> --- a/fs/ceph/mds_client.h
> +++ b/fs/ceph/mds_client.h
> @@ -534,6 +534,9 @@ struct ceph_mds_client {
>  	struct list_head        snap_empty;
>  	int			num_snap_realms;
>  	spinlock_t              snap_empty_lock;  /* protect
> snap_empty */
> +	spinlock_t              wait_list_lock;   /* protect
> waiting_for_map
> +						   * and s_waiting
> lists
> +						   */
>  
>  	u64                    last_tid;      /* most recent mds
> request */
>  	atomic64_t             oldest_tid;    /* oldest incomplete
> mds request,