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SubjectRe: [PATCH] Don't apply for write lock on tasklist_lock if parent doesn't ptrace other processes
I am not surpized perf blaims tasklist, but I am really surpized this patch
adds 10% improvement...

On 07/21, Roland McGrath wrote:
>
> > > @@ -331,6 +331,9 @@ void exit_ptrace(struct task_struct *tra
> > > struct task_struct *p, *n;
> > > LIST_HEAD(ptrace_dead);
> > >
> > > + if (list_empty(&tracer->ptraced))
> > > + return;
> > > +
> > > write_lock_irq(&tasklist_lock);
> > > list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) {
> > > if (__ptrace_detach(tracer, p))
>
> I think we may have tried that before. Oleg can tell us if it's really
> safe vs a race with PTRACE_TRACEME or something like that.

Yes, this can race with ptrace_traceme(). Without tasklist_lock in
exit_ptrace(), it is possible that ptrace_traceme() starts __ptrace_link()
before it sees PF_EXITING, and completes before the result of list_add()
is visible to the exiting parent. tasklist acts as a barrier.

So, this list_empty() check needs taskslit at least for reading. But, we
are going to take it for writing right after exit_ptrace() returns, afaics
we can add this fastpatch check for free.

Uncompiled/untested.

Oleg.

kernel/ptrace.c | 10 +++++++---
kernel/exit.c | 3 ++-
2 files changed, 9 insertions(+), 4 deletions(-)
--- x/kernel/ptrace.c
+++ x/kernel/ptrace.c
@@ -324,26 +324,30 @@ 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.
*/
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)
--- x/kernel/exit.c
+++ x/kernel/exit.c
@@ -771,9 +771,10 @@ static void forget_original_parent(struc
struct task_struct *p, *n, *reaper;
LIST_HEAD(dead_children);

+ write_lock_irq(&tasklist_lock);
+
exit_ptrace(father);

- write_lock_irq(&tasklist_lock);
reaper = find_new_reaper(father);

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


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