Messages in this thread | | | Date | Thu, 23 Feb 2012 13:40:14 -0600 (CST) | From | Christoph Lameter <> | Subject | Re: [RFC][PATCH] fix move/migrate_pages() race on task struct |
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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?
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