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Date: Thu, 18 Jun 2009 12:07:16 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: dhowells@...hat.com, jmorris@...ei.org, roland@...hat.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rework/fix is_single_threaded()
On Thu, 16 Apr 2009 16:54:44 +0200
Oleg Nesterov <oleg@...hat.com> wrote:
> On 04/16, Oleg Nesterov wrote:
> >
> > Suppose we have a process P which shares ->mm with "task" (the argument), so
> > we should return "false".
> >
> > P does clone(CLONE_VM) and exits. rcu_read_lock() can't guarantee we will
> > see the new task with the same ->mm. And without ->mmap_sem P can call
> > exit_mm() and set P->mm = NULL.
> >
> > Hmm. But we can just add a barrier?
> >
> > bool is_single_threaded(struct task_struct *task)
> > {
> > struct mm_struct *mm = task->mm;
> > struct task_struct *p, *t;
> > bool ret;
> >
> > if (atomic_read(&task->signal->live) != 1)
> > return false;
> >
> > if (atomic_read(&mm->mm_users) == 1)
> > return true;
> >
> > ret = false;
> > rcu_read_lock();
> > for_each_process(p) {
> > if (unlikely(p->flags & PF_KTHREAD))
> > continue;
> > if (unlikely(p == task->group_leader))
> > continue;
> >
> > t = p;
> > do {
> > if (unlikely(t->mm == mm))
> > goto found;
> > if (likely(t->mm))
> > break;
> >
> > /* !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
> > t->mm == NULL. Perhaps it had the same ->mm ?
> > If t has forked CLONE_VM task and called exit_mm(),
> > make sure next_thread() or for_each_process()->next_task()
> > will see it.
> > !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
> > */
> > smp_rmb();
>
> Sorry, forgot to mention...
>
> But what if P does clone(CLONE_VM), exits, and for_each_process/while_each_thread
> doesn't see it? IOW, what if we already see the result of list_del_rcu() ?
>
> I think, in that case we must also see the result of clone()->list_add_tail_rcu()
> which has a barrier, so we are safe.
>
> Hmm. I feel this all has a simpler explanation, or I missed something...
>
It appears that this patch is rather stuck. Should I drop it?
From: Oleg Nesterov <oleg@...hat.com>
- Fix the comment, is_single_threaded(p) actually means that nobody shares
->mm with p.
I think this helper should be renamed, and it should not have arguments.
With or without this patch it must not be used unless p == current,
otherwise we can't safely use p->signal or p->mm.
- "if (atomic_read(&p->signal->count) != 1)" is not right when we have a
zombie group leader, use signal->live instead.
- Add PF_KTHREAD check to skip kernel threads which may borrow p->mm,
otherwise we can return the wrong "false".
- Use for_each_process() instead of do_each_thread(), all threads must use
the same ->mm.
- Use down_write(mm->mmap_sem) + rcu_read_lock() instead of tasklist_lock
to iterate over the process list. If there is another CLONE_VM process
it can't pass exit_mm() which takes the same mm->mmap_sem. We can miss
a freshly forked CLONE_VM task, but this doesn't matter because we must
see its parent and return false.
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Cc: David Howells <dhowells@...hat.com>
Cc: James Morris <jmorris@...ei.org>
Cc: Roland McGrath <roland@...hat.com>
Cc: Stephen Smalley <sds@...ho.nsa.gov>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---
lib/is_single_threaded.c | 62 +++++++++++++++++++++----------------
1 file changed, 36 insertions(+), 26 deletions(-)
diff -puN lib/is_single_threaded.c~rework-fix-is_single_threaded lib/is_single_threaded.c
--- a/lib/is_single_threaded.c~rework-fix-is_single_threaded
+++ a/lib/is_single_threaded.c
@@ -12,34 +12,44 @@
#include <linux/sched.h>
-/**
- * is_single_threaded - Determine if a thread group is single-threaded or not
- * @p: A task in the thread group in question
- *
- * This returns true if the thread group to which a task belongs is single
- * threaded, false if it is not.
+/*
+ * Returns true if the task does not share ->mm with another thread/process.
*/
-bool is_single_threaded(struct task_struct *p)
+bool is_single_threaded(struct task_struct *task)
{
- struct task_struct *g, *t;
- struct mm_struct *mm = p->mm;
-
- if (atomic_read(&p->signal->count) != 1)
- goto no;
-
- if (atomic_read(&p->mm->mm_users) != 1) {
- read_lock(&tasklist_lock);
- do_each_thread(g, t) {
- if (t->mm == mm && t != p)
- goto no_unlock;
- } while_each_thread(g, t);
- read_unlock(&tasklist_lock);
+ struct mm_struct *mm = task->mm;
+ struct task_struct *p, *t;
+ bool ret;
+
+ might_sleep();
+
+ if (atomic_read(&task->signal->live) != 1)
+ return false;
+
+ if (atomic_read(&mm->mm_users) == 1)
+ return true;
+
+ ret = false;
+ down_write(&mm->mmap_sem);
+ rcu_read_lock();
+ for_each_process(p) {
+ if (unlikely(p->flags & PF_KTHREAD))
+ continue;
+ if (unlikely(p == task->group_leader))
+ continue;
+
+ t = p;
+ do {
+ if (unlikely(t->mm == mm))
+ goto found;
+ if (likely(t->mm))
+ break;
+ } while_each_thread(p, t);
}
+ ret = true;
+found:
+ rcu_read_unlock();
+ up_write(&mm->mmap_sem);
- return true;
-
-no_unlock:
- read_unlock(&tasklist_lock);
-no:
- return false;
+ return ret;
}
_
--
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