5.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: John Fastabend <john.fastabend@xxxxxxxxx>
commit 42a84a8cd0ff0cbff5a4595e1304c4567a30267d upstream.
Now that we rely on synchronize_rcu and call_rcu waiting to
exit perempt-disable regions (NAPI) lets update the comments
to reflect this.
Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
Signed-off-by: John Fastabend <john.fastabend@xxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: Björn Töpel <bjorn.topel@xxxxxxxxx>
Acked-by: Song Liu <songliubraving@xxxxxx>
Link: https://lore.kernel.org/bpf/1580084042-11598-2-git-send-email-john.fastabend@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/bpf/devmap.c | 21 +++++++++++----------
1 file changed, 11 insertions(+), 10 deletions(-)
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -210,10 +210,12 @@ static void dev_map_free(struct bpf_map
/* At this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
* so the programs (can be more than one that used this map) were
- * disconnected from events. Wait for outstanding critical sections in
- * these programs to complete. The rcu critical section only guarantees
- * no further reads against netdev_map. It does __not__ ensure pending
- * flush operations (if any) are complete.
+ * disconnected from events. The following synchronize_rcu() guarantees
+ * both rcu read critical sections complete and waits for
+ * preempt-disable regions (NAPI being the relevant context here) so we
+ * are certain there will be no further reads against the netdev_map and
+ * all flush operations are complete. Flush operations can only be done
+ * from NAPI context for this reason.
*/
spin_lock(&dev_map_lock);
@@ -518,12 +520,11 @@ static int dev_map_delete_elem(struct bp
return -EINVAL;
/* Use call_rcu() here to ensure any rcu critical sections have
- * completed, but this does not guarantee a flush has happened
- * yet. Because driver side rcu_read_lock/unlock only protects the
- * running XDP program. However, for pending flush operations the
- * dev and ctx are stored in another per cpu map. And additionally,
- * the driver tear down ensures all soft irqs are complete before
- * removing the net device in the case of dev_put equals zero.
+ * completed as well as any flush operations because call_rcu
+ * will wait for preempt-disable region to complete, NAPI in this
+ * context. And additionally, the driver tear down ensures all
+ * soft irqs are complete before removing the net device in the
+ * case of dev_put equals zero.
*/
old_dev = xchg(&dtab->netdev_map[k], NULL);
if (old_dev)
