Christoph Lameter <cl@xxxxxxxxx> writes: > On Thu, 23 Feb 2012, Dave Hansen wrote: > >> > We may at this point be getting a reference to a task struct from another >> > process not only from the current process (where the above procedure is >> > valid). You rightly pointed out that the slab rcu free mechanism allows a >> > free and a reallocation within the RCU period. >> >> I didn't _mean_ to point that out, but I think I realize what you're >> talking about. What we have before this patch is this: >> >> rcu_read_lock(); >> task = pid ? find_task_by_vpid(pid) : current; > > We take a refcount here on the mm ... See the code. We could simply take a > refcount on the task as well if this is considered safe enough. If we have > a refcount on the task then we do not need the refcount on the mm. Thats > was your approach... > >> rcu_read_unlock(); > >> > Is that a real difference or are you just playing with words? >> >> I think we're talking about two different things: >> 1. does RCU protect the pid->task lookup sufficiently? > > I dont know Yes. See below. >> 2. Can the task simply go away in the move/migrate_pages() calls? > > The task may go away but we need the mm to stay for migration. > That is why a refcount is taken on the mm. > > The bug in migrate_pages() is that we do a rcu_unlock and a rcu_lock. If > we drop those then we should be safe if the use of a task pointer within a > rcu section is safe without taking a refcount. Yes the user of a task_struct pointer found via a userspace pid is valid for the life of an rcu critical section, and the bug is indeed that we drop the rcu_lock and somehow expect the task to remain valid. The guarantee comes from release_task. In release_task we call __exit_signal which calls __unhash_process, and then we call delayed_put_task to guarantee that the task lives until the end of the rcu interval. In migrate_pages we have a lot of task accesses outside of the rcu critical section, and without a reference count on task. I tell you the truth trying to figure out what that code needs to be correct if task != current makes my head hurt. I think we need to grab a reference on task_struct, to stop the task from going away, and in addition we need to hold task_lock. To keep task->mm from changing (see exec_mmap). But we can't do that and sleep so I think the entire function needs to be rewritten, and the need for task deep in the migrate_pages path needs to be removed as even with the reference count held we can race with someone calling exec. The only easy fix I see is to add: if (pid) return -EINVAL; Then we are working with current and only current change it's mm making things much, much, much simpler. Eric -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>