Messages in this thread | | | Date | Thu, 23 Feb 2012 12:04:23 -0800 | From | Dave Hansen <> | Subject | Re: [RFC][PATCH] fix move/migrate_pages() race on task struct |
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On 02/23/2012 11:40 AM, Christoph Lameter wrote: > 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.
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; rcu_read_unlock();
task->foo;
So, the task at task->foo time is neither RCU-protected nor protected by having a reference. I changed it to:
rcu_read_lock(); task = pid ? find_task_by_vpid(pid) : current; get_task_struct(task); rcu_read_unlock();
task->foo;
That keeps task from being freed. But, as you point out
> 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.
Agreed, this patch would not fix such an issue.
I think this also implies that stuff like get_task_pid() is broken, along with virtually all of the users of find_task_by_vpid(). Eric, any thoughts on this?
> 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?
I think we're talking about two different things: 1. does RCU protect the pid->task lookup sufficiently? 2. Can the task simply go away in the move/migrate_pages() calls?
I think we're on the same page now.
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