[PATCH] process accounting: take original leader's start_time in non-leader exec

The only record we have of the real-time age of a process, regardless of
execs it's done, is start_time.  When a non-leader thread exec, the
original start_time of the process is lost.  Things looking at the
real-time age of the process are fooled, for example the process accounting
record when the process finally dies.  This change makes the oldest
start_time stick around with the process after a non-leader exec.  This way
the association between PID and start_time is kept constant, which seems
correct to me.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 4d38ad0b70d6..3234a0c32d54 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -678,6 +678,18 @@ static int de_thread(struct task_struct *tsk)
                 while (leader->exit_state != EXIT_ZOMBIE)
                         yield();
 
+                /*
+                 * The only record we have of the real-time age of a
+                 * process, regardless of execs it's done, is start_time.
+                 * All the past CPU time is accumulated in signal_struct
+                 * from sister threads now dead.  But in this non-leader
+                 * exec, nothing survives from the original leader thread,
+                 * whose birth marks the true age of this process now.
+                 * When we take on its identity by switching to its PID, we
+                 * also take its birthdate (always earlier than our own).
+                 */
+                current->start_time = leader->start_time;
+
                 spin_lock(&leader->proc_lock);
                 spin_lock(&current->proc_lock);
                 proc_dentry1 = proc_pid_unhash(current);