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From: Oleg Nesterov on 29 Jul 2010 11:20
(replaces ptrace-dont-run-write_locktasklist_lock-if-the-parent-doesnt-ptrace-other-processes.patch)

exit_ptrace() takes tasklist_lock unconditionally. We need this lock
to avoid the race with ptrace_traceme(), it acts as a barrier.

Change its caller, forget_original_parent(), to call exit_ptrace()
under tasklist_lock. Change exit_ptrace() to drop and reacquire this
lock if needed.

This allows us to add the fastpath list_empty(ptraced) check. In the
likely no-tracees case exit_ptrace() just returns and we avoid the
lock() + unlock() sequence.

"Zhang, Yanmin" <yanmin_zhang(a)linux.intel.com> suggested to add this
check, and he reports that this change adds about 11% improvement in
some tests.

Suggested-and-tested-by: "Zhang, Yanmin" <yanmin_zhang(a)linux.intel.com>
Signed-off-by: Oleg Nesterov <oleg(a)redhat.com>
---

kernel/ptrace.c | 12 +++++++++---
kernel/exit.c | 7 +++++--
2 files changed, 14 insertions(+), 5 deletions(-)

--- 35-rc3/kernel/ptrace.c~exit_ptrace_fastpath_check 2010-05-28 13:41:41.000000000 +0200
+++ 35-rc3/kernel/ptrace.c 2010-07-29 16:37:13.000000000 +0200
@@ -324,26 +324,32 @@ int ptrace_detach(struct task_struct *ch
}

/*
- * Detach all tasks we were using ptrace on.
+ * Detach all tasks we were using ptrace on. Called with tasklist held
+ * for writing, and returns with it held too. But note it can release
+ * and reacquire the lock.
*/
void exit_ptrace(struct task_struct *tracer)
{
struct task_struct *p, *n;
LIST_HEAD(ptrace_dead);

- write_lock_irq(&tasklist_lock);
+ if (likely(list_empty(&tracer->ptraced)))
+ return;
+
list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) {
if (__ptrace_detach(tracer, p))
list_add(&p->ptrace_entry, &ptrace_dead);
}
- write_unlock_irq(&tasklist_lock);

+ write_unlock_irq(&tasklist_lock);
BUG_ON(!list_empty(&tracer->ptraced));

list_for_each_entry_safe(p, n, &ptrace_dead, ptrace_entry) {
list_del_init(&p->ptrace_entry);
release_task(p);
}
+
+ write_lock_irq(&tasklist_lock);
}

int ptrace_readdata(struct task_struct *tsk, unsigned long src, char __user *dst, int len)
--- 35-rc3/kernel/exit.c~exit_ptrace_fastpath_check 2010-05-28 13:41:41.000000000 +0200
+++ 35-rc3/kernel/exit.c 2010-07-29 16:38:37.000000000 +0200
@@ -771,9 +771,12 @@ static void forget_original_parent(struc
struct task_struct *p, *n, *reaper;
LIST_HEAD(dead_children);

- exit_ptrace(father);
-
write_lock_irq(&tasklist_lock);
+ /*
+ * Note that exit_ptrace() and find_new_reaper() might
+ * drop tasklist_lock and reacquire it.
+ */
+ exit_ptrace(father);
reaper = find_new_reaper(father);

list_for_each_entry_safe(p, n, &father->children, sibling) {

--
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Prev: writeback: prevent sync livelock with the sync_after timestamp
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