[PATCH net v3 04/11] list: Move on_list_rcu() to list.h and add on_list() also

David Howells
2026-03-26T13:19:14Z

Unfortunately, list_empty() is not usable with an entry that has been
removed from a list with list_del_rcu() as ->next must be left pointing at
the following entry so as not to break traversal under RCU.

Solve this by moving on_list_rcu() from AppArmor to linux/list.h, and
turning it into an inline function.

Also add an on_list() counterpart (functionally, this is just an antonym
for list_empty()), but the name looks less awkward when applied to a
non-head element. We probably don't want to use on_list_rcu() generally
because it requires an extra check as ->prev is set differently in the two
cases.

Further, rename the on_list() function in the Designware usb2 drd ip driver
to dwc2_on_list() to free up the original name.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
cc: John Johansen <john.johansen@canonical.com>
cc: Minas Harutyunyan <hminas@synopsys.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: apparmor@lists.ubuntu.com
cc: linux-usb@vger.kernel.org
cc: netdev@vger.kernel.org
cc: stable@kernel.org
---
drivers/usb/dwc2/gadget.c | 6 +++---
include/linux/list.h | 26 ++++++++++++++++++++++++++
security/apparmor/include/policy.h | 2 --
3 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/dwc2/gadget.c b/drivers/usb/dwc2/gadget.c
index d216e26c787b..04b6aef8ac13 100644
--- a/drivers/usb/dwc2/gadget.c
+++ b/drivers/usb/dwc2/gadget.c
@@ -4306,11 +4306,11 @@ static int dwc2_hsotg_ep_disable_lock(struct usb_ep *ep)
}

/**
- * on_list - check request is on the given endpoint
+ * dwc2_on_list - check request is on the given endpoint
* @ep: The endpoint to check.
* @test: The request to test if it is on the endpoint.
*/
-static bool on_list(struct dwc2_hsotg_ep *ep, struct dwc2_hsotg_req *test)
+static bool dwc2_on_list(struct dwc2_hsotg_ep *ep, struct dwc2_hsotg_req *test)
{
struct dwc2_hsotg_req *req, *treq;

@@ -4338,7 +4338,7 @@ static int dwc2_hsotg_ep_dequeue(struct usb_ep *ep, struct usb_request *req)

spin_lock_irqsave(&hs->lock, flags);

- if (!on_list(hs_ep, hs_req)) {
+ if (!dwc2_on_list(hs_ep, hs_req)) {
spin_unlock_irqrestore(&hs->lock, flags);
return -EINVAL;
}
diff --git a/include/linux/list.h b/include/linux/list.h
index 00ea8e5fb88b..d224e7210d1b 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -381,6 +381,32 @@ static inline int list_empty(const struct list_head *head)
return READ_ONCE(head->next) == head;
}

+/**
+ * on_list - Test whether an entry is on a list.
+ * @entry: The entry to check
+ *
+ * Test whether an entry is on a list. Safe to use on an entry initialised
+ * with INIT_LIST_HEAD() or LIST_HEAD() or removed with things like
+ * list_del_init(). Not safe for use with list_del() or list_del_rcu().
+ */
+static inline bool on_list(const struct list_head *entry)
+{
+ return !list_empty(entry);
+}
+
+/**
+ * on_list_rcu - Test whether an entry is on a list (RCU-del safe).
+ * @entry: The entry to check
+ *
+ * Test whether an entry is on a list. Safe to use on an entry initialised
+ * with INIT_LIST_HEAD() or LIST_HEAD() or removed with things like
+ * list_del_init(). Also safe for use with list_del() or list_del_rcu().
+ */
+static inline bool on_list_rcu(const struct list_head *entry)
+{
+ return !list_empty(entry) && entry->prev != LIST_POISON2;
+}
+
/**
* list_del_init_careful - deletes entry from list and reinitialize it.
* @entry: the element to delete from the list.
diff --git a/security/apparmor/include/policy.h b/security/apparmor/include/policy.h
index 3895f8774a3f..c3697c23bbed 100644
--- a/security/apparmor/include/policy.h
+++ b/security/apparmor/include/policy.h
@@ -57,8 +57,6 @@ extern const char *const aa_profile_mode_names[];

#define profile_is_stale(_profile) (label_is_stale(&(_profile)->label))

-#define on_list_rcu(X) (!list_empty(X) && (X)->prev != LIST_POISON2)
-
/* flags in the dfa accept2 table */
enum dfa_accept_flags {
ACCEPT_FLAG_OWNER = 1,