netdev - Re: [PATCH net-next 2/3] bpf: permit multiple bpf attachments for a single perf event

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Date:   Thu, 26 Oct 2017 09:42:57 -0700
From:   Yonghong Song <yhs@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <rostedt@...dmis.org>, <ast@...com>, <daniel@...earbox.net>,
        <kafai@...com>, <netdev@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH net-next 2/3] bpf: permit multiple bpf attachments for a
 single perf event

On 10/26/17 6:56 AM, Peter Zijlstra wrote:
> On Mon, Oct 23, 2017 at 10:58:04AM -0700, Yonghong Song wrote:
>> This patch enables multiple bpf attachments for a
>> kprobe/uprobe/tracepoint single trace event.
> 
> This forgets to explain _why_ this is a good thing to do.

Before this patch, each perf tracepoint event (tp_event)
can only attach one bpf program. Each tp_event is internally
identifiable through a config ID, through which perf_event_open
associates to a particular tp_event.

Only one ID is associated with each kernel tracepoint. We have
use cases that an application already attached a bpf program to
a particular kernel tracepoint (e.g., block:block_rq_issue).
Another unrelated application tries to attach its own bpf program
to the same tracepoint but failed. This patch removed such
a limitation and permits more than one bpf programs attaching
to the same tracepoint.

Strictly speaking, kprobe/uprobe does not need this as users
can always create a new tp_event attaching to the same kprobe/uprobe.
However, since in kernel kprobe/uprobe/tracepoint shared the same
tp_event infrastructure, adding this support avoid code/data_structure
multiplexing with just a little non-configuration runtime overhead
(going through a one-element pointer array vs. a pointer).

Sorry about missing this piece in the commit message.
Will try to do better next time.

> 
>> +static DEFINE_MUTEX(bpf_event_mutex);
>> +
>> +int perf_event_attach_bpf_prog(struct perf_event *event,
>> +			   struct bpf_prog *prog)
>> +{
>> +	struct bpf_prog_array __rcu *old_array;
>> +	struct bpf_prog_array *new_array;
>> +	int ret;
>> +
>> +	mutex_lock(&bpf_event_mutex);
>> +
>> +	if (event->prog)
>> +		return -EEXIST;
>> +
>> +	old_array = rcu_dereference_protected(event->tp_event->prog_array,
>> +					      lockdep_is_held(&bpf_event_mutex));
> 
> Since all modifications to prog_array are serialized by this one mutex;
> you don't need rcu_dereference() here, there are no possible ordering
> problems.

Yes, will fix this in a subsequent patch.

>> +	ret = bpf_prog_array_copy(old_array, NULL, prog, &new_array);
>> +	if (ret < 0)
>> +		goto out;
>> +
>> +	/* set the new array to event->tp_event and set event->prog */
>> +	event->prog = prog;
>> +	rcu_assign_pointer(event->tp_event->prog_array, new_array);
>> +
>> +	if (old_array)
>> +		bpf_prog_array_free(old_array);
>> +
>> +out:
> 
> Its customary to call that unlock:

Yes. Will fix it in a subsequent patch.

> 
>> +	mutex_unlock(&bpf_event_mutex);
>> +	return ret;
>> +}