Re: [patch 0/9] Fix various __task_cred related invalid RCU assumptions

Paul E. McKenney
2009-12-10T05:34:58Z

On Thu, Dec 10, 2009 at 06:13:51AM +0100, Peter Zijlstra wrote:
> On Wed, 2009-12-09 at 19:15 -0800, Linus Torvalds wrote:
> >
> > On Wed, 9 Dec 2009, Paul E. McKenney wrote:
> >
> > > On Thu, Dec 10, 2009 at 12:52:46AM -0000, Thomas Gleixner wrote:
> > > > While auditing the read_lock(&tasklist_lock) sites for a possible
> > > > conversion to rcu-read_lock() I stumbled over an unprotected user of
> > > > __task_cred in kernel/sys.c
> > > >
> > > > That caused me to audit all the __task_cred usage sites except in
> > > > kernel/exit.c.
> > > >
> > > > Most of the usage sites are correct, but some of them trip over
> > > > invalid assumptions about the protection which is given by RCU.
> > > >
> > > > - spinlocked/preempt_disabled regions are equivalent to rcu_read_lock():
> > > >
> > > > That's wrong. RCU does not guarantee that.
> > > >
> > > > It has been that way due to implementation details and it still is
> > > > valid for CONFIG_TREE_PREEMPT_RCU=n, but there is no guarantee that
> > > > this will be the case forever.
> > >
> > > To back this up, item #2 from Documentation/RCU/checklist.txt says:
> >
> > Hmm. This seems to be a difference that the tree-RCU things introduced,
> > no? I wonder if we have other areas where we just knew that a spinlock
> > would make an rcu read-lock unnecessary (which used to be true..)
>
> That failed being true when we merged PREEMPT_RCU,.. which was a long
> time ago.

Ah -- I have a related lockdep question. Is there a primitive that says
whether or not the current task holds at least one lock of any type?
If so, I would like to make rcu_dereference() do at least a little crude
checking for this problem.

Thanx, Paul