[PATCH review 11/11] mnt: Honor MNT_LOCKED when detaching mounts

Al Viro viro at ZenIV.linux.org.uk
Sun Jan 11 02:00:30 UTC 2015

On Sat, Jan 10, 2015 at 05:51:48AM +0000, Al Viro wrote:
> On Fri, Jan 09, 2015 at 11:32:47PM -0600, Eric W. Biederman wrote:
> > I don't believe rcu anything in this function itself buys you anything,
> > but structuring this primitive so that it can be called from an rcu list
> > traversal seems interesting.  
> ???
> Without RCU, what would prevent it being freed right under us?
> The whole point is to avoid pinning it down - as it is, we can have
> several processes call ->kill() on the same object.  The first one
> would end up doing cleanup, the rest would wait *without* *affecting*
> *fs_pin* *lifetime*.
> Note that I'm using autoremove there for wait.func(), then in the wait
> loop I check (without locks) wait.task_list being empty.  It is racy;
> deliberately so.  All I really care about in there is checking that
> wait.func has not been called until after rcu_read_lock().  If that is
> true, we know that p->wait hadn't been woken until that point, i.e.
> p hadn't reached rcu delay on the way to being freed until after our
> rcu_read_lock().  Ergo, it can't get freed until we do rcu_read_unlock()
> and we can safely take p->wait.lock.
> RCU is very much relevant there.

FWIW, I've just pushed a completely untested tree in #experimental-fs_pin;
it definitely will be reordered, etc., probably with quite a few of the
patches from the beginning of your series mixed in, but the current tree
in there should show at least what I'm aiming at.

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