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Message-ID: <20100722090524.GA6647@redhat.com>
Date: Thu, 22 Jul 2010 11:05:24 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Roland McGrath <roland@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
LKML <linux-kernel@...r.kernel.org>, andi.kleen@...el.com,
stable@...nel.org
Subject: Re: [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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