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coredump: zap_threads: comments && use while_each_thread() 

No changes in fs/exec.o

The for_each_process() loop in zap_threads() is very subtle, it is not
clear why we don't race with fork/exit/exec.  Add the fat comment.

Also, change the code to use while_each_thread().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2008-07-25 01:47:31 -07:00 committed by Linus Torvalds
parent 2b201a9edd
commit e4901f92a8
1 changed files with 32 additions and 7 deletions
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39
fs/exec.c
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@ -1517,7 +1517,7 @@ static void zap_process(struct task_struct *start)
sigaddset(&t->pending.signal, SIGKILL);
signal_wake_up(t, 1);
}
} while ((t = next_thread(t)) != start);
} while_each_thread(start, t);
}
static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
@ -1539,7 +1539,36 @@ static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
if (atomic_read(&mm->mm_users) == mm->core_waiters + 1)
goto done;
/*
* We should find and kill all tasks which use this mm, and we should
* count them correctly into mm->core_waiters. We don't take tasklist
* lock, but this is safe wrt:
*
* fork:
* None of sub-threads can fork after zap_process(leader). All
* processes which were created before this point should be
* visible to zap_threads() because copy_process() adds the new
* process to the tail of init_task.tasks list, and lock/unlock
* of ->siglock provides a memory barrier.
*
* do_exit:
* The caller holds mm->mmap_sem. This means that the task which
* uses this mm can't pass exit_mm(), so it can't exit or clear
* its ->mm.
*
* de_thread:
* It does list_replace_rcu(&leader->tasks, &current->tasks),
* we must see either old or new leader, this does not matter.
* However, it can change p->sighand, so lock_task_sighand(p)
* must be used. Since p->mm != NULL and we hold ->mmap_sem
* it can't fail.
*
* Note also that "g" can be the old leader with ->mm == NULL
* and already unhashed and thus removed from ->thread_group.
* This is OK, __unhash_process()->list_del_rcu() does not
* clear the ->next pointer, we will find the new leader via
* next_thread().
*/
rcu_read_lock();
for_each_process(g) {
if (g == tsk->group_leader)
@ -1549,17 +1578,13 @@ static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
do {
if (p->mm) {
if (p->mm == mm) {
/*
* p->sighand can't disappear, but
* may be changed by de_thread()
*/
lock_task_sighand(p, &flags);
zap_process(p);
unlock_task_sighand(p, &flags);
}
break;
}
} while ((p = next_thread(p)) != g);
} while_each_thread(g, p);
}
rcu_read_unlock();
done: