Re: [PATCH 1/5] ceph: convert oldest_tid to atomic64_t

[Viacheslav Dubeyko <slava@dubeyko.com>]

On Mon, 2026-07-13 at 17:46 +0800, Xiubo Li via B4 Relay wrote:
> From: Xiubo Li <xiubo.li@clyso.com>
> 
> The oldest_client_tid sent in the MDS request header is advisory:
> a stale value is harmless -- at worst the MDS may resend a reply
> we already have, or skip one we still need (which will just be
> retried).  With the plain u64 read, however, the compiler is free
> to split or cache the load, which is undefined behaviour when the
> write side runs under mdsc->mutex on a different CPU.
> 
> Convert mdsc->oldest_tid from u64 to atomic64_t so that reads
> are guaranteed to be single-copy atomic on all architectures.
> This removes the one remaining reason __prepare_send_request()
> and __send_request() needed to be called under mdsc->mutex, so
> drop those comments as well.
> 
> All write sites (__register_request, __unregister_request) still
> run under mdsc->mutex, so use atomic64_set() for clarity.
> 
> Signed-off-by: Xiubo Li <xiubo.li@clyso.com>
> ---
>  fs/ceph/mds_client.c | 21 ++++++++-------------
>  fs/ceph/mds_client.h |  2 +-
>  2 files changed, 9 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> index 9f84ef2ac6e4..98d0a5baff70 100644
> --- a/fs/ceph/mds_client.c
> +++ b/fs/ceph/mds_client.c
> @@ -1235,8 +1235,9 @@ static void __register_request(struct
> ceph_mds_client *mdsc,
>  	if (!req->r_mnt_idmap)
>  		req->r_mnt_idmap = &nop_mnt_idmap;
>  
> -	if (mdsc->oldest_tid == 0 && req->r_op !=
> CEPH_MDS_OP_SETFILELOCK)
> -		mdsc->oldest_tid = req->r_tid;
> +	if (atomic64_read(&mdsc->oldest_tid) == 0 &&
> +	    req->r_op != CEPH_MDS_OP_SETFILELOCK)
> +		atomic64_set(&mdsc->oldest_tid, req->r_tid);
>  
>  	if (dir) {
>  		struct ceph_inode_info *ci = ceph_inode(dir);
> @@ -1257,14 +1258,14 @@ static void __unregister_request(struct
> ceph_mds_client *mdsc,
>  	/* Never leave an unregistered request on an unsafe list! */
>  	list_del_init(&req->r_unsafe_item);
>  
> -	if (req->r_tid == mdsc->oldest_tid) {
> +	if (req->r_tid == atomic64_read(&mdsc->oldest_tid)) {
>  		struct rb_node *p = rb_next(&req->r_node);
> -		mdsc->oldest_tid = 0;
> +		atomic64_set(&mdsc->oldest_tid, 0);
>  		while (p) {
>  			struct ceph_mds_request *next_req =
>  				rb_entry(p, struct ceph_mds_request,
> r_node);
>  			if (next_req->r_op !=
> CEPH_MDS_OP_SETFILELOCK) {
> -				mdsc->oldest_tid = next_req->r_tid;
> +				atomic64_set(&mdsc->oldest_tid,
> next_req->r_tid);
>  				break;
>  			}
>  			p = rb_next(p);
> @@ -1693,7 +1694,7 @@ create_session_full_msg(struct ceph_mds_client
> *mdsc, int op, u64 seq)
>  	ceph_encode_32(&p, 0);
>  
>  	/* version == 7, oldest_client_tid */
> -	ceph_encode_64(&p, mdsc->oldest_tid);
> +	ceph_encode_64(&p, atomic64_read(&mdsc->oldest_tid));
>  
>  	msg->front.iov_len = p - msg->front.iov_base;
>  	msg->hdr.front_len = cpu_to_le32(msg->front.iov_len);
> @@ -2759,7 +2760,7 @@ static struct ceph_mds_request
> *__get_oldest_req(struct ceph_mds_client *mdsc)
>  
>  static inline  u64 __get_oldest_tid(struct ceph_mds_client *mdsc)
>  {
> -	return mdsc->oldest_tid;
> +	return atomic64_read(&mdsc->oldest_tid);
>  }
>  
>  #if IS_ENABLED(CONFIG_FS_ENCRYPTION)
> @@ -3438,9 +3439,6 @@ static void complete_request(struct
> ceph_mds_client *mdsc,
>  	complete_all(&req->r_completion);
>  }
>  
> -/*
> - * called under mdsc->mutex
> - */
>  static int __prepare_send_request(struct ceph_mds_session *session,
>  				  struct ceph_mds_request *req,
>  				  bool drop_cap_releases)
> @@ -3555,9 +3553,6 @@ static int __prepare_send_request(struct
> ceph_mds_session *session,
>  	return 0;
>  }
>  
> -/*
> - * called under mdsc->mutex
> - */
>  static int __send_request(struct ceph_mds_session *session,
>  			  struct ceph_mds_request *req,
>  			  bool drop_cap_releases)
> diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
> index 731d6ad04956..3b614b5df18c 100644
> --- a/fs/ceph/mds_client.h
> +++ b/fs/ceph/mds_client.h
> @@ -532,7 +532,7 @@ struct ceph_mds_client {
>  	spinlock_t              snap_empty_lock;  /* protect
> snap_empty */
>  
>  	u64                    last_tid;      /* most recent mds
> request */
> -	u64                    oldest_tid;    /* oldest incomplete
> mds request,
> +	atomic64_t             oldest_tid;    /* oldest incomplete
> mds request,
>  						 excluding
> setfilelock requests */
>  	struct rb_root         request_tree;  /* pending mds
> requests */
>  	struct delayed_work    delayed_work;  /* delayed work */

Looks good.

Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>

Thanks,
Slava.