[PATCH v4 18/21] afs: Fix lack of locking around modifications of net->cells_dyn_ino

[David Howells <dhowells@redhat.com>]

Fix the lack of locking around modifications of net->cells_dyn_ino by
taking net->cells_lock exclusively.  This also requires to cell to be
removed from net->cells_dyn_ino in afs_destroy_cell_work() rather than in
afs_cell_destroy() as the latter runs in RCU cleanup context and sleeping
locks cannot be taken there.

Fixes: 1d0b929fc070 ("afs: Change dynroot to create contents on demand")
Closes: https://sashiko.dev/#/patchset/20260618074903.2374756-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
---
 fs/afs/cell.c    | 8 +++++++-
 fs/afs/dynroot.c | 2 +-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/afs/cell.c b/fs/afs/cell.c
index fbb8a43aa7cd..9d8937ae24e2 100644
--- a/fs/afs/cell.c
+++ b/fs/afs/cell.c
@@ -205,8 +205,10 @@ static struct afs_cell *afs_alloc_cell(struct afs_net *net,
 	cell->dns_source = vllist->source;
 	cell->dns_status = vllist->status;
 	smp_store_release(&cell->dns_lookup_count, 1); /* vs source/status */
+	down_write(&net->cells_lock);
 	ret = idr_alloc_cyclic(&net->cells_dyn_ino, cell,
 			       2, INT_MAX / 2, GFP_KERNEL);
+	up_write(&net->cells_lock);
 	if (ret < 0)
 		goto error;
 	atomic_inc(&net->cells_outstanding);
@@ -579,7 +581,6 @@ static void afs_cell_destroy(struct rcu_head *rcu)
 	afs_put_vlserverlist(net, rcu_access_pointer(cell->vl_servers));
 	afs_unuse_cell(cell->alias_of, afs_cell_trace_unuse_alias);
 	key_put(cell->anonymous_key);
-	idr_remove(&net->cells_dyn_ino, cell->dynroot_ino);
 	kfree(cell->name - 1);
 	kfree(cell);
 
@@ -594,6 +595,11 @@ static void afs_destroy_cell_work(struct work_struct *work)
 	afs_see_cell(cell, afs_cell_trace_destroy);
 	timer_delete_sync(&cell->management_timer);
 	cancel_work_sync(&cell->manager);
+
+	down_write(&cell->net->cells_lock);
+	idr_remove(&cell->net->cells_dyn_ino, cell->dynroot_ino);
+	up_write(&cell->net->cells_lock);
+
 	call_rcu(&cell->rcu, afs_cell_destroy);
 }
 
diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index 1d5e33bc7502..6e3c8c691ba9 100644
--- a/fs/afs/dynroot.c
+++ b/fs/afs/dynroot.c
@@ -278,7 +278,7 @@ static struct dentry *afs_lookup_atcell(struct inode *dir, struct dentry *dentry
 }
 
 /*
- * Transcribe the cell database into readdir content under the RCU read lock.
+ * Transcribe the cell database into readdir content under net->cells_lock.
  * Each cell produces two entries, one prefixed with a dot and one not.
  */
 static int afs_dynroot_readdir_cells(struct afs_net *net, struct dir_context *ctx)