netdev - Re: [PATCH bpf-next 07/12] bpf: Add a hint to allocated objects.

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Message-ID: <CAADnVQ+ROd__AXmHcUTy3j8zYL7zr6brA3swS9P6OmN_2BwcrQ@mail.gmail.com>
Date: Fri, 23 Jun 2023 20:42:09 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Hou Tao <houtao@...weicloud.com>
Cc: Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	David Vernet <void@...ifault.com>, "Paul E. McKenney" <paulmck@...nel.org>, Tejun Heo <tj@...nel.org>, 
	rcu@...r.kernel.org, Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 07/12] bpf: Add a hint to allocated objects.

On Fri, Jun 23, 2023 at 8:28 PM Hou Tao <houtao@...weicloud.com> wrote:
>
> >                */
> > -             obj = __llist_del_first(&c->free_by_rcu_ttrace);
> > +             obj = llist_del_first(&c->free_by_rcu_ttrace);
> According to the comments in llist.h, when there are concurrent
> llist_del_first() and llist_del_all() operations, locking is needed.

Good question.
1. When only one cpu is doing llist_del_first() locking is not needed.
 This is the case here. Only this cpu is doing llist_del_first() from this 'c'.
2. The comments doesn't mention it, but llist_del_first() is ok on
multiple cpus if ABA problem is addressed by other means.

PS
please trim your replies.