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authorOleg Nesterov <oleg@tv-sign.ru>2008-07-25 01:47:31 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2008-07-25 10:53:38 -0700
commite4901f92a8dbe843e76651a50f7a2a6dd3d53474 (patch)
tree4cfd5edd17fcf1fe5e4372611c561928a99107d0 /fs
parent2b201a9eddf509e8e935b45e573648e36f4b623f (diff)
downloadlinux-2.6.34-ux500-e4901f92a8dbe843e76651a50f7a2a6dd3d53474.tar.gz
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>
Diffstat (limited to 'fs')
-rw-r--r--fs/exec.c39
1 files changed, 32 insertions, 7 deletions
diff --git a/fs/exec.c b/fs/exec.c
index e41aef0fb35..af249af4cca 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -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: