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[2003:dc:6f02:b200:230:64ff:fe74:809]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-482fbb2794dsm5480678f8f.25.2026.08.28.10.45.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Aug 2026 10:45:14 -0700 (PDT) From: Max Kellermann To: idryomov@gmail.com, amarkuze@redhat.com, xiubo.li@clyso.com, ceph-devel@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Max Kellermann Subject: [PATCH v3 3/3] ceph: don't unregister an MDS session before removing its caps Date: Fri, 28 Aug 2026 19:45:04 +0200 Message-ID: <20260828174504.1247038-4-max.kellermann@ionos.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260828174504.1247038-1-max.kellermann@ionos.com> References: <20260828174504.1247038-1-max.kellermann@ionos.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit handle_session() removed the session from mdsc->sessions[] at the very top of `CEPH_SESSION_CLOSE` handling, before taking `s_mutex`. Between session unregistration and remove_session_caps(), the MDS rank has no registered session while the old session still owns all caps it was granted. Any concurrent filesystem operation may walk into that and the next __do_request() call registers a new session for this rank. Once it is open and the MDS issues caps, ceph_fill_inode() calls ceph_add_cap(), which looks caps up by rank, not by session identity, finding old caps linked to the old session. The list_move_tail() call then moves the cap object to the new session, which is already a bad thing to do. Since it doesn't decrement `old_session->s_nr_caps`, this will quickly run into a BUG() instead of crashing: kernel BUG at fs/ceph/mds_client.c:1959! Internal error: Oops - BUG: 00000000f2000800 [#1] SMP [...] Workqueue: ceph-msgr ceph_con_workfn pstate: 20400009 (nzCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : remove_session_caps+0x2bc/0x2d8 lr : remove_session_caps+0x74/0x2d8 [...] Call trace: remove_session_caps+0x2bc/0x2d8 (P) mds_dispatch+0xf48/0x1b60 ceph_con_process_message+0x74/0xa0 ceph_con_v1_try_read+0x3a0/0x1510 ceph_con_workfn+0x260/0x460 process_one_work+0x168/0x3b8 worker_thread+0x1bc/0x3a0 kthread+0x118/0x1e0 ret_from_fork+0x10/0x20 That's BUG_ON(session->s_nr_caps > 0). I was able to reproduce this reliably by delaying the close and starting I/O during the delay. This patch keeps the session registered with `CEPH_MDS_SESSION_CLOSED`. New requests will be put on the `s_waiting` list where they will be resumed on the new session. Signed-off-by: Max Kellermann --- v1->v2: skip CLOSED sessions in check_new_map() v2->v3: split session pinning and stale-map guards into two preparatory patches; rework CLOSED-session and export-target teardown handling Signed-off-by: Max Kellermann --- fs/ceph/caps.c | 3 + fs/ceph/mds_client.c | 138 ++++++++++++++++++++++++++++++++++++++----- 2 files changed, 127 insertions(+), 14 deletions(-) diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c index d7283fb54cec..b21d0a9ac324 100644 --- a/fs/ceph/caps.c +++ b/fs/ceph/caps.c @@ -4195,6 +4195,9 @@ static void handle_cap_export(struct inode *inode, struct ceph_mds_caps *ex, } new_cap = ceph_get_cap(mdsc, NULL); } else { + if (tsession == ERR_PTR(-EAGAIN)) + return; + WARN_ON(1); tsession = NULL; target = -1; diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c index 03809328e4aa..ec8ee95cad6e 100644 --- a/fs/ceph/mds_client.c +++ b/fs/ceph/mds_client.c @@ -1733,6 +1733,30 @@ static int __open_session(struct ceph_mds_client *mdsc, return 0; } +/* Is this rank still occupied by a session being torn down? */ +static bool __mds_rank_closing(struct ceph_mds_client *mdsc, int mds) +{ + return mds < mdsc->max_sessions && mdsc->sessions[mds] && + mdsc->sessions[mds]->s_state == CEPH_MDS_SESSION_CLOSED; +} + +/* + * Wait until the rank is no longer occupied by a CLOSED session. + * + * The caller must hold mdsc->mutex. The mutex is dropped while sleeping and + * held again on return. Another session may occupy the rank by then. The + * wait also ends when shutdown starts, so the caller must check + * mdsc->stopping before proceeding. + */ +static void wait_for_mds_rank_not_closing(struct ceph_mds_client *mdsc, + int mds) +{ + wait_event_cmd(mdsc->session_close_wq, + !__mds_rank_closing(mdsc, mds) || mdsc->stopping, + mutex_unlock(&mdsc->mutex), + mutex_lock(&mdsc->mutex)); +} + /* * open sessions for any export targets for the given mds * @@ -1750,6 +1774,16 @@ __open_export_target_session(struct ceph_mds_client *mdsc, int target) if (IS_ERR(session)) return session; } + if (session->s_state == CEPH_MDS_SESSION_CLOSED) { + /* + * handle_session() is currently closing this session; + * it stays registered until its caps are gone. Do + * not return it to our caller because we don't want + * it to attach new caps to it. + */ + ceph_put_mds_session(session); + return ERR_PTR(-EAGAIN); + } if (session->s_state == CEPH_MDS_SESSION_NEW || session->s_state == CEPH_MDS_SESSION_CLOSING) { ret = __open_session(mdsc, session); @@ -1769,7 +1803,19 @@ ceph_mdsc_open_export_target_session(struct ceph_mds_client *mdsc, int target) doutc(cl, "to mds%d\n", target); mutex_lock(&mdsc->mutex); - session = __open_export_target_session(mdsc, target); + for (;;) { + session = __open_export_target_session(mdsc, target); + if (session != ERR_PTR(-EAGAIN) || mdsc->stopping) + break; + + /* + * Keep the exported cap on its old session until the + * target rank is vacant. Dropping it here can discard a + * dirty auth cap; handle_session() wakes us after removing + * the old target session's caps and unregistering it. + */ + wait_for_mds_rank_not_closing(mdsc, target); + } mutex_unlock(&mdsc->mutex); return session; @@ -4492,8 +4538,20 @@ static void handle_session(struct ceph_mds_session *session, ceph_metric_bind_session(mdsc, session); } if (op == CEPH_SESSION_CLOSE) { + /* + * Pin the session for the rest of this function. The + * __unregister_session() call is deferred until after + * remove_session_caps() below, or else other + * processes may find caps still assigned to this + * session while working with a new session object. + */ ceph_get_mds_session(session); - __unregister_session(mdsc, session); + + if (session->s_state == CEPH_MDS_SESSION_RECONNECTING) + pr_info_client(cl, "mds%d reconnect denied\n", + session->s_mds); + + session->s_state = CEPH_MDS_SESSION_CLOSED; } /* FIXME: this ttl calculation is generous */ session->s_ttl = jiffies + HZ*mdsc->mdsmap->m_session_autoclose; @@ -4551,12 +4609,24 @@ static void handle_session(struct ceph_mds_session *session, break; case CEPH_SESSION_CLOSE: - if (session->s_state == CEPH_MDS_SESSION_RECONNECTING) - pr_info_client(cl, "mds%d reconnect denied\n", - session->s_mds); - session->s_state = CEPH_MDS_SESSION_CLOSED; cleanup_session_requests(mdsc, session); remove_session_caps(session); + + /* + * Now that all caps are removed, it is safe release + * the MDS rank and allow other processes to create a + * new session object. + * + * A concurrent ceph_mdsc_close_sessions() or + * check_new_map() may have unregistered the session + * already, so check __verify_registered_session() + * first. + */ + mutex_lock(&mdsc->mutex); + if (!__verify_registered_session(mdsc, session)) + __unregister_session(mdsc, session); + mutex_unlock(&mdsc->mutex); + wake = 2; /* for good measure */ wake_up_all(&mdsc->session_close_wq); break; @@ -5802,6 +5872,10 @@ static void check_new_map(struct ceph_mds_client *mdsc, s = __ceph_lookup_mds_session(mdsc, i); if (!s) continue; + if (s->s_state == CEPH_MDS_SESSION_CLOSED) { + ceph_put_mds_session(s); + continue; + } oldstate = ceph_mdsmap_get_state(oldmap, i); newstate = ceph_mdsmap_get_state(newmap, i); @@ -5814,18 +5888,24 @@ static void check_new_map(struct ceph_mds_client *mdsc, if (i >= newmap->possible_max_rank) { /* force close session for stopped mds */ - __unregister_session(mdsc, s); - __wake_requests(mdsc, &s->s_waiting); + s->s_state = CEPH_MDS_SESSION_CLOSED; mutex_unlock(&mdsc->mutex); mutex_lock(&s->s_mutex); cleanup_session_requests(mdsc, s); remove_session_caps(s); + + mutex_lock(&mdsc->mutex); + if (!__verify_registered_session(mdsc, s)) + __unregister_session(mdsc, s); + mutex_unlock(&mdsc->mutex); mutex_unlock(&s->s_mutex); - ceph_put_mds_session(s); + wake_up_all(&mdsc->session_close_wq); mutex_lock(&mdsc->mutex); + __wake_requests(mdsc, &s->s_waiting); + ceph_put_mds_session(s); if (mdsc->mdsmap->m_epoch != map_epoch) return; kick_requests(mdsc, i); @@ -5844,6 +5924,16 @@ static void check_new_map(struct ceph_mds_client *mdsc, ceph_put_mds_session(s); return; } + if (s->s_state == CEPH_MDS_SESSION_CLOSED) { + /* + * handle_session() set state=CLOSED + * in the mutex gap above and is tearing it + * down + */ + mutex_unlock(&s->s_mutex); + ceph_put_mds_session(s); + continue; + } ceph_con_close(&s->s_con); mutex_unlock(&s->s_mutex); s->s_state = CEPH_MDS_SESSION_RESTARTING; @@ -5902,6 +5992,9 @@ static void check_new_map(struct ceph_mds_client *mdsc, * Only open and reconnect sessions that don't exist yet. */ for (i = 0; i < newmap->possible_max_rank; i++) { + if (mdsc->stopping) + return; + /* * In case the import MDS is crashed just after * the EImportStart journal is flushed, so when @@ -5927,6 +6020,19 @@ static void check_new_map(struct ceph_mds_client *mdsc, * reconnection request in up:reconnect state. */ s = __ceph_lookup_mds_session(mdsc, i); + if (s && s->s_state == CEPH_MDS_SESSION_CLOSED) { + /* + * Wait for handle_session() to remove the caps and + * unregister this session, so the reconnect below + * uses a fresh session on the now vacant rank + */ + ceph_put_mds_session(s); + wait_for_mds_rank_not_closing(mdsc, i); + if (mdsc->stopping || + mdsc->mdsmap->m_epoch != map_epoch) + return; + s = NULL; + } if (likely(!s)) { s = __open_export_target_session(mdsc, i); if (IS_ERR(s)) { @@ -7087,9 +7193,7 @@ static void mds_peer_reset(struct ceph_connection *con) * Snapshot session state with READ_ONCE, then revalidate under * mdsc->mutex before acting. The subsequent mdsc->mutex * section rechecks s_state to catch concurrent transitions, so - * the lockless snapshot here is safe. s->s_mutex is taken - * separately for cleanup after unregistration, which avoids - * introducing a new s->s_mutex + mdsc->mutex nesting. + * the lockless snapshot here is safe. */ session_state = READ_ONCE(s->s_state); @@ -7114,18 +7218,24 @@ static void mds_peer_reset(struct ceph_connection *con) ceph_get_mds_session(s); s->s_state = CEPH_MDS_SESSION_CLOSED; - __unregister_session(mdsc, s); - __wake_requests(mdsc, &s->s_waiting); mutex_unlock(&mdsc->mutex); mutex_lock(&s->s_mutex); cleanup_session_requests(mdsc, s); remove_session_caps(s); + + /* Keep the rank occupied until all old-session caps are gone. */ + mutex_lock(&mdsc->mutex); + if (!__verify_registered_session(mdsc, s)) + __unregister_session(mdsc, s); + mutex_unlock(&mdsc->mutex); + mutex_unlock(&s->s_mutex); wake_up_all(&mdsc->session_close_wq); mutex_lock(&mdsc->mutex); + __wake_requests(mdsc, &s->s_waiting); kick_requests(mdsc, s->s_mds); mutex_unlock(&mdsc->mutex); -- 2.47.3