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;
+}
