linux-kernel - Re: [RFC][PATCH] fix move/migrate

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Date:	Thu, 23 Feb 2012 13:40:14 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] fix move/migrate_pages() race on task struct

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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