netdev - Re: [PATCH v2 bpf-next 02/10] bpf: allocate ID for bpf

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Date:   Tue, 28 Apr 2020 10:31:20 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, andrii.nakryiko@...il.com, kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 02/10] bpf: allocate ID for bpf_link

On Mon, Apr 27, 2020 at 10:49:36PM -0700, Andrii Nakryiko wrote:
> +int bpf_link_settle(struct bpf_link_primer *primer)
> +{
> +	/* make bpf_link fetchable by ID */
> +	WRITE_ONCE(primer->link->id, primer->id);

what does WRITE_ONCE serve here?
bpf_link_settle can only be called at the end of attach.
If attach is slow than parallel get_fd_by_id can get an new FD
instance for link with zero id.
In such case deref of link->id will race with above assignment?
But I don't see READ_ONCE in patch 3.
It's under link_idr_lock there.
How about grabbing link_idr_lock here as well ?
otherwise it's still racy since WRITE_ONCE is not paired.

The mix of spin_lock_irqsave(&link_idr_lock)
and spin_lock_bh(&link_idr_lock) looks weird.
We do the same for map_idr because maps have complicated freeing logic,
but prog_idr is consistent.
If you see the need for irqsave variant then please use it in all cases.