On Thu, 23 Feb 2012, Dave Hansen wrote: > > Hmmm isnt the race still there between the determination of the task and > > the get_task_struct()? You would have to verify after the get_task_struct > > that this is really the task we wanted to avoid the race. > > It's true that selecting a task by pid is inherently racy. What that > code does is ensure that the task you've got current has 'pid', but not > ensure that 'pid' has never represented another task. But, that's what > we do everywhere else in the kernel; there's not much better that we can do. 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. The effect is that the task struct could be pointing to a task with another pid that what we were looking for and therefore migrate_pages could subsequently be operating on a totally different process. The patch does not fix that race so far. I think you have to verify that the pid of the task matches after you took the refcount in order to be safe. If it does not match then abort. > Maybe "race" is the wrong word for what we've got here. It's a lack of > a refcount being taken. Is that a real difference or are you just playing with words? -- 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>