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

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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: 0536b85239 ("xdp: Simplify devmap cleanup")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/1580084042-11598-2-git-send-email-john.fastabend@gmail.com
This commit is contained in:
John Fastabend 2020-01-26 16:14:00 -08:00 committed by Daniel Borkmann
parent 90435a7891
commit 42a84a8cd0
1 changed files with 11 additions and 10 deletions
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21
kernel/bpf/devmap.c
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@ -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)