netdev - Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read

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Date:   Wed, 08 Jan 2020 22:35:10 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        bjorn.topel@...il.com, bpf@...r.kernel.org, toke@...hat.com
Cc:     netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net
Subject: Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required
 rcu_read_{un}lock()

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@...il.com>
> > ---
> >   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?

Ah OK thanks for catching this. So its a strange requirement from
virto_net to need read_lock like this. Quickly scanned the drivers
and seems its the only one.

I think the best path forward is to fix virtio_net so it doesn't
need rcu_read_lock() here then the locking is much cleaner IMO.

I'll send a v2 and either move the xdp enabled check (the piece
using the rcu_read_lock) into a bitmask flag or push the
rcu_read_lock() into virtio_net so its clear that this is a detail
of virtio_net and not a general thing. FWIW I don't think the
rcu_read_lock is actually needed in the virtio_net case anymore
either but pretty sure the rcu_dereference will cause an rcu
splat. Maybe there is another annotation we can use. I'll dig
into it tomorrow. Thanks

> 
> Toshiaki Makita