On 2020/01/09 15:01, Toshiaki Makita wrote:
On 2020/01/09 6:35, John Fastabend wrote:Now that we depend on rcu_call() and synchronize_rcu() to also wait for preempt_disabled region to complete the rcu read critical section in __dev_map_flush() is no longer relevant. These originally ensured the map reference was safe while a map was also being free'd. But flush by new rules can only be called from preempt-disabled NAPI context. The synchronize_rcu from the map free path and the rcu_call from the delete path will ensure the reference here is safe. So lets remove the rcu_read_lock and rcu_read_unlock pair to avoid any confusion around how this is being protected. If the rcu_read_lock was required it would mean errors in the above logic and the original patch would also be wrong. Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup") Signed-off-by: John Fastabend <john.fastabend@xxxxxxxxx> --- kernel/bpf/devmap.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c index f0bf525..0129d4a 100644 --- a/kernel/bpf/devmap.c +++ b/kernel/bpf/devmap.c @@ -378,10 +378,8 @@ void __dev_map_flush(void) struct list_head *flush_list = this_cpu_ptr(&dev_map_flush_list); struct xdp_bulk_queue *bq, *tmp; - rcu_read_lock(); list_for_each_entry_safe(bq, tmp, flush_list, flush_node) bq_xmit_all(bq, XDP_XMIT_FLUSH); - rcu_read_unlock();I introduced this lock because some drivers have assumption that .ndo_xdp_xmit() is called under RCU. (commit 86723c864063) Maybe devmap deletion logic does not need this anymore, but is it OK to drivers?
More references for the driver problem: - Lockdep splat in virtio_net: https://lists.openwall.net/netdev/2019/04/24/282 - Discussion for fix: https://lists.openwall.net/netdev/2019/04/25/234 Toshiaki Makita
