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 > 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. -- 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>