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

Peter Zijlstra
2009-12-10T05:12:43Z

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.