Q: PTRACE_ATTACH && -EINTR

[Oleg Nesterov <oleg@redhat.com>]

I just realized that ->cred_exec_mutex added a user-visible change
which may confuse user-space.

	ptrace_attach:

		retval = mutex_lock_interruptible(&task->cred_exec_mutex);
		if (retval  < 0)
			goto out;

This doesn't look good, we return -EINTR. Suppose that strace tries to
attach to all sub-threads and ptrace(PTRACE_ATTACH) returns -EINTR just
because the already traced thread sends SIGCHLD. Or tracer's sub-thread
does recalc_sigpending_and_wake().

I think we should at least do

		retval = -ERESTARTSYS;
		if (mutex_lock_interruptible(&task->cred_exec_mutex))
			goto out;

Or even -ERESTARTNOINTR ? Or just mutex_lock() ?

Or ignore this problem since nobody complained?

Oleg.