[PATCH 3/3] ptrace: kill may_ptrace_stop()

[Oleg Nesterov <oleg@tv-sign.ru>]

Now that ptrace_stop() can't sleep if the tracee was SIGKILL'ed, we don't
need the special mm->core_waiters check in may_ptrace_stop(). It was needed
to prevent the deadlock with the tracer with the same ->mm which waits for
us in coredump_wait()->wait_for_completion(&startup_done), and the the
unkillable tracee waits for PTRACE_CONT from the tracer.

Without this check may_ptrace_stop() becomes trivial and can be removed.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 26-rc2/kernel/signal.c~	2008-06-15 16:15:46.000000000 +0400
+++ 26-rc2/kernel/signal.c	2008-06-15 17:51:50.000000000 +0400
@@ -1472,26 +1472,6 @@ static void do_notify_parent_cldstop(str
 	spin_unlock_irqrestore(&sighand->siglock, flags);
 }
 
-static inline int may_ptrace_stop(void)
-{
-	if (!likely(current->ptrace & PT_PTRACED))
-		return 0;
-	/*
-	 * Are we in the middle of do_coredump?
-	 * If so and our tracer is also part of the coredump stopping
-	 * is a deadlock situation, and pointless because our tracer
-	 * is dead so don't allow us to stop.
-	 * If SIGKILL was already sent before the caller unlocked
-	 * ->siglock we must see ->core_waiters != 0. Otherwise it
-	 * is safe to enter schedule().
-	 */
-	if (unlikely(current->mm->core_waiters) &&
-	    unlikely(current->mm == current->parent->mm))
-		return 0;
-
-	return 1;
-}
-
 /*
  * This must be called with current->sighand->siglock held.
  *
@@ -1536,7 +1516,7 @@ static void ptrace_stop(int exit_code, i
 	__set_current_state(TASK_TRACED);
 	spin_unlock_irq(&current->sighand->siglock);
 	read_lock(&tasklist_lock);
-	if (may_ptrace_stop()) {
+	if (likely(current->ptrace & PT_PTRACED)) {
 		do_notify_parent_cldstop(current, CLD_TRAPPED);
 		read_unlock(&tasklist_lock);
 		schedule();