netdevsim: psp: serialize calls to nsim_psp_uninit()

The debugfs write handler, nsim_psp_rereg_write(), can race against
nsim_destroy() and against itself, causing nsim_psp_uninit() to run
more than once concurrently. Two complementary changes serialize all
callers:

1. Delete the psp_rereg debugfs file from nsim_psp_uninit() before
   doing the actual teardown. debugfs_remove() drains any in-flight
   writers and prevents new ones from starting.

2. Add a mutex around the body of nsim_psp_rereg_write() so that two
   concurrent userspace writers cannot both enter the teardown path
   at once.

The teardown work itself is moved into a new __nsim_psp_uninit() that
the rereg handler calls under the mutex, while the public
nsim_psp_uninit() wraps it with the debugfs_remove()/mutex_destroy()
pair so nsim_destroy() doesn't have to know about the psp internals.

Fixes: f857478 ("netdevsim: a basic test PSP implementation")
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260505-psd-rcu-v1-2-a8f69ec1ab96@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Daniel Zahka

drivers/net/netdevsim/netdevsim.h

@@ -121,6 +121,8 @@ struct netdevsim {
 		u64_stats_t tx_bytes;
 		struct u64_stats_sync syncp;
 		struct psp_dev *dev;
+		struct dentry *rereg;
+		struct mutex rereg_lock;
 		u32 spi;
 		u32 assoc_cnt;
 	} psp;

drivers/net/netdevsim/psp.c

@@ -209,25 +209,34 @@ static struct psp_dev_caps nsim_psp_caps = {
 	.assoc_drv_spc = sizeof(void *),
 };
 
-void nsim_psp_uninit(struct netdevsim *ns)
+static void __nsim_psp_uninit(struct netdevsim *ns)
 {
 	if (!IS_ERR(ns->psp.dev))
 		psp_dev_unregister(ns->psp.dev);
 	WARN_ON(ns->psp.assoc_cnt);
 }
 
+void nsim_psp_uninit(struct netdevsim *ns)
+{
+	debugfs_remove(ns->psp.rereg);
+	mutex_destroy(&ns->psp.rereg_lock);
+	__nsim_psp_uninit(ns);
+}
+
 static ssize_t
 nsim_psp_rereg_write(struct file *file, const char __user *data, size_t count,
 		     loff_t *ppos)
 {
 	struct netdevsim *ns = file->private_data;
 	int err;
 
-	nsim_psp_uninit(ns);
+	mutex_lock(&ns->psp.rereg_lock);
+	__nsim_psp_uninit(ns);
 
 	ns->psp.dev = psp_dev_create(ns->netdev, &nsim_psp_ops,
 				     &nsim_psp_caps, ns);
 	err = PTR_ERR_OR_ZERO(ns->psp.dev);
+	mutex_unlock(&ns->psp.rereg_lock);
 	return err ?: count;
 }
 
@@ -249,6 +258,8 @@ int nsim_psp_init(struct netdevsim *ns)
 	if (err)
 		return err;
 
-	debugfs_create_file("psp_rereg", 0200, ddir, ns, &nsim_psp_rereg_fops);
+	mutex_init(&ns->psp.rereg_lock);
+	ns->psp.rereg = debugfs_create_file("psp_rereg", 0200, ddir, ns,
+					    &nsim_psp_rereg_fops);
 	return 0;
 }