ptrace: use bit_waitqueue for TRAPPING instead of wait_chldexit

ptracer->signal->wait_chldexit was used to wait for TRAPPING; however,
->wait_chldexit was already complicated with waker-side filtering
without adding TRAPPING wait on top of it.  Also, it unnecessarily
made TRAPPING clearing depend on the current ptrace relationship - if
the ptracee is detached, wakeup is lost.

There is no reason to use signal->wait_chldexit here.  We're just
waiting for JOBCTL_TRAPPING bit to clear and given the relatively
infrequent use of ptrace, bit_waitqueue can serve it perfectly.

This patch makes JOBCTL_TRAPPING wait use bit_waitqueue instead of
signal->wait_chldexit.

-v2: Use JOBCTL_*_BIT macros instead of ilog2() as suggested by Linus.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 9ab91c516c3f..172a4c79f12c 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -272,8 +272,7 @@ static void task_clear_jobctl_trapping(struct task_struct *task)
 {
 	if (unlikely(task->jobctl & JOBCTL_TRAPPING)) {
 		task->jobctl &= ~JOBCTL_TRAPPING;
-		__wake_up_sync_key(&task->parent->signal->wait_chldexit,
-				   TASK_UNINTERRUPTIBLE, 1, task);
+		wake_up_bit(&task->jobctl, JOBCTL_TRAPPING_BIT);
 	}
 }