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

From: Linus Torvalds
Date: Wed Dec 09 2009 - 22:17:14 EST




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..)

Linus
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