Commit 42a84a8c authored by John Fastabend's avatar John Fastabend Committed by Daniel Borkmann
Browse files

bpf, xdp: Update devmap comments to reflect napi/rcu usage 



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: 0536b852 ("xdp: Simplify devmap cleanup")
Signed-off-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarBjörn Töpel <bjorn.topel@intel.com>
Acked-by: default avatarSong Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/1580084042-11598-2-git-send-email-john.fastabend@gmail.com
parent 90435a78

kernel/bpf/devmap.c

+11 −10
Original line number Diff line number Diff line
@@ -190,10 +190,12 @@ static void dev_map_free(struct bpf_map *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);
@@ -503,12 +505,11 @@ static int dev_map_delete_elem(struct bpf_map *map, void *key) 
		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)

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