[PATCH bpf-next v2 1/8] bpf: offload: don't require rtnl for dev list manipulation

[Jakub Kicinski]

We don't need the RTNL lock for all operations on offload state.
We only need to hold it around ndo calls.  The device offload
initialization doesn't require it.  The soon-to-come querying
of the offload info will only need it partially.  We will also
be able to remove the waitqueue in following patches.

Use struct rw_semaphore because map offload will require sleeping
with the semaphore held for read.

Suggested-by: Kirill Tkhai <ktk...@virtuozzo.com>
Signed-off-by: Jakub Kicinski <jakub.kicin...@netronome.com>
Reviewed-by: Quentin Monnet <quentin.mon...@netronome.com>
---
v2:
 - use dev_get_by_index_rcu() instead of implicit lock dependencies;
 - use DECLARE_RWSEM() instead of init_rwsem() (Kirill).
---
 kernel/bpf/offload.c | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/offload.c b/kernel/bpf/offload.c
index 8455b89d1bbf..f049073a37e6 100644
--- a/kernel/bpf/offload.c
+++ b/kernel/bpf/offload.c
@@ -20,8 +20,12 @@
 #include <linux/netdevice.h>
 #include <linux/printk.h>
 #include <linux/rtnetlink.h>
+#include <linux/rwsem.h>
 
-/* protected by RTNL */
+/* Protects bpf_prog_offload_devs and offload members of all progs.
+ * RTNL lock cannot be taken when holding this lock.
+ */
+static DECLARE_RWSEM(bpf_devs_lock);
 static LIST_HEAD(bpf_prog_offload_devs);
 
 int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr)
@@ -43,19 +47,30 @@ int bpf_prog_offload_init(struct bpf_prog *prog, union 
bpf_attr *attr)
        offload->prog = prog;
        init_waitqueue_head(&offload->verifier_done);
 
-       rtnl_lock();
-       offload->netdev = __dev_get_by_index(net, attr->prog_ifindex);
+       rcu_read_lock();
+       offload->netdev = dev_get_by_index_rcu(net, attr->prog_ifindex);
        if (!offload->netdev) {
-               rtnl_unlock();
-               kfree(offload);
-               return -EINVAL;
+               rcu_read_unlock();
+               goto err_free;
        }
+       dev_hold(offload->netdev);
+       rcu_read_unlock();
 
+       down_write(&bpf_devs_lock);
+       if (offload->netdev->reg_state != NETREG_REGISTERED)
+               goto err_unlock;
        prog->aux->offload = offload;
        list_add_tail(&offload->offloads, &bpf_prog_offload_devs);
-       rtnl_unlock();
+       dev_put(offload->netdev);
+       up_write(&bpf_devs_lock);
 
        return 0;
+err_unlock:
+       up_write(&bpf_devs_lock);
+       dev_put(offload->netdev);
+err_free:
+       kfree(offload);
+       return -EINVAL;
 }
 
 static int __bpf_offload_ndo(struct bpf_prog *prog, enum bpf_netdev_command 
cmd,
@@ -126,7 +141,9 @@ void bpf_prog_offload_destroy(struct bpf_prog *prog)
        wake_up(&offload->verifier_done);
 
        rtnl_lock();
+       down_write(&bpf_devs_lock);
        __bpf_prog_offload_destroy(prog);
+       up_write(&bpf_devs_lock);
        rtnl_unlock();
 
        kfree(offload);
@@ -181,11 +198,13 @@ static int bpf_offload_notification(struct notifier_block 
*notifier,
                if (netdev->reg_state != NETREG_UNREGISTERING)
                        break;
 
+               down_write(&bpf_devs_lock);
                list_for_each_entry_safe(offload, tmp, &bpf_prog_offload_devs,
                                         offloads) {
                        if (offload->netdev == netdev)
                                __bpf_prog_offload_destroy(offload->prog);
                }
+               up_write(&bpf_devs_lock);
                break;
        default:
                break;
-- 
2.15.1