ptrace: don't call PTRACE_INTERRUPT many times

According to the commit message in the kernel git, a new trap flag
JOBCTL_TRAP_INTERRUPT is added, which is set on PTRACE_INTERRUPT and
cleared when any trap happens.

Currently we have a race window beetwen PTRACE_CONT and
PTRACE_INTERRUPT. If a process stops before PTRACE_INTERRUPT,
the flag JOBCTL_TRAP_INTERRUPT will be set once again.

https://bugzilla.openvz.org/show_bug.cgi?id=2569Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

ptrace.c

@@ -88,13 +88,14 @@ int seize_task(pid_t pid, pid_t ppid, pid_t *pgid, pid_t *sid)
 				pid, ppid, ps.ppid);
 		goto err;
 	}
-try_again:
+
 	ret = ptrace(PTRACE_INTERRUPT, pid, NULL, NULL);
 	if (ret < 0) {
 		pr_perror("SEIZE %d: can't interrupt task", pid);
 		goto err;
 	}
 
+try_again:
 	ret = wait4(pid, &status, __WALL, NULL);
 	if (ret < 0) {
 		pr_perror("SEIZE %d: can't wait task", pid);