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kernel: fix is_single_threaded

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- 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@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Morris <jmorris@namei.org>
This commit is contained in:
Oleg Nesterov 2009-07-17 09:09:36 +10:00 committed by James Morris
parent 713c0ecdb8
commit d2e3ee9b29
1 changed files with 34 additions and 24 deletions
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58
lib/is_single_threaded.c
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@ -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;
struct mm_struct *mm = task->mm;
struct task_struct *p, *t;
bool ret;
if (atomic_read(&p->signal->count) != 1)
goto no;
might_sleep();
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);
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;
}