[PATCH] perf: Remove confusing comment

[Peter Zijlstra]

Since commit: 321027c1fe77 ("perf/core: Fix concurrent
sys_perf_event_open() vs. 'move_group' race") the code looks like
(assuming move_group==1):

  gctx = __perf_event_ctx_lock_double(group_leader, ctx);

  perf_remove_from_context(group_leader, 0);
  list_for_each_entry(sibling, &group_leader->sibling_list, group_entry) {
	perf_remove_from_context(sibling, 0);
	put_ctx(gctx);
  }

  /* ... */

  /* misleading comment about how this is the last reference */
  put_ctx(gctx);

  perf_event_ctx_unlock(group_leader, gctx);

What that 'last' put_ctx() does is drop @group_leader's reference on
gctx after having dropped all its potential sibling references.

But the thing is that __perf_event_ctx_lock_double() returns with a
reference _and_ a held lock, and perf_event_ctx_unlock() unlocks that
lock and drops that reference. Therefore that put_ctx() cannot be the
'last' of anything, nor is there an unbalance in puts.

To reduce confusion, remove the comment and place the put_ctx() next
to the remove_from_context() call.

Reported-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/events/core.c |    8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9957,6 +9957,7 @@ SYSCALL_DEFINE5(perf_event_open,
 		 * of swizzling perf_event::ctx.
 		 */
 		perf_remove_from_context(group_leader, 0);
+		put_ctx(gctx);
 
 		list_for_each_entry(sibling, &group_leader->sibling_list,
 				    group_entry) {
@@ -9995,13 +9996,6 @@ SYSCALL_DEFINE5(perf_event_open,
 		perf_event__state_init(group_leader);
 		perf_install_in_context(ctx, group_leader, group_leader->cpu);
 		get_ctx(ctx);
-
-		/*
-		 * Now that all events are installed in @ctx, nothing
-		 * references @gctx anymore, so drop the last reference we have
-		 * on it.
-		 */
-		put_ctx(gctx);
 	}
 
 	/*