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, 8 insertions, 2 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 0c37d99..7f05f3a 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -25,6 +25,12 @@
 #include <linux/hw_breakpoint.h>
 
 
+static int ptrace_trapping_sleep_fn(void *flags)
+{
+	schedule();
+	return 0;
+}
+
 /*
  * ptrace a task: make the debugger its new parent and
  * move it to the ptrace list.
@@ -270,8 +276,8 @@ unlock_creds:
 	mutex_unlock(&task->signal->cred_guard_mutex);
 out:
 	if (!retval)
-		wait_event(current->signal->wait_chldexit,
-			   !(task->jobctl & JOBCTL_TRAPPING));
+		wait_on_bit(&task->jobctl, JOBCTL_TRAPPING_BIT,
+			    ptrace_trapping_sleep_fn, TASK_UNINTERRUPTIBLE);
 	return retval;
 }