[PATCH 13/16] ptrace: Document why ptrace_setoptions does not need a lock

[Eric W. Biederman]

The functions that change ->ptrace are: ptrace_attach, ptrace_traceme,
ptrace_init_task, __ptrace_unlink, ptrace_setoptions.

Except for ptrace_setoptions all of the places where ->ptrace is
modified hold tasklist_lock for write, and either the tracee or the
tracer is modifies ->ptrace.

When ptrace_setoptions is called the tracee has been frozen with
ptrace_freeze_traced, and most be explicitly unfrozen by the tracer
before it can do anything.  As ptrace_setoption is run in the tracer
there can be no contention by the simple fact that the tracee can't
run.

Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
---
 kernel/ptrace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index d0527b6e2b29..fbadd2f21f09 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -689,7 +689,10 @@ static int ptrace_setoptions(struct task_struct *child, unsigned long data)
 	if (ret)
 		return ret;
 
-	/* Avoid intermediate state when all opts are cleared */
+	/*
+	 * With a frozen tracee, only the tracer modifies ->ptrace.
+	 * Avoid intermediate state when all opts are cleared.
+	 */
 	flags = child->ptrace;
 	flags &= ~(PTRACE_O_MASK << PT_OPT_FLAG_SHIFT);
 	flags |= (data << PT_OPT_FLAG_SHIFT);
-- 
2.35.3