netdev - Re: [RFC PATCH bpf-next 08/16] bpf: add task and task/file targets

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Message-ID: <20200410032223.esp46oxtpegextxn@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 9 Apr 2020 20:22:23 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, kernel-team@...com
Subject: Re: [RFC PATCH bpf-next 08/16] bpf: add task and task/file targets

On Wed, Apr 08, 2020 at 04:25:29PM -0700, Yonghong Song wrote:
> +
> +	spin_lock(&files->file_lock);
> +	for (; sfd < files_fdtable(files)->max_fds; sfd++) {
> +		struct file *f;
> +
> +		f = fcheck_files(files, sfd);
> +		if (!f)
> +			continue;
> +
> +		*fd = sfd;
> +		get_file(f);
> +		spin_unlock(&files->file_lock);
> +		return f;
> +	}
> +
> +	/* the current task is done, go to the next task */
> +	spin_unlock(&files->file_lock);
> +	put_files_struct(files);

I think spin_lock is unnecessary.
It's similarly unnecessary in bpf_task_fd_query().
Take a look at proc_readfd_common() in fs/proc/fd.c.
It only needs rcu_read_lock() to iterate fd array.