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SubjectRe: Documentation/credentials.txt
Quoting Paul E. McKenney (paulmck@linux.vnet.ibm.com):
> On Fri, Apr 23, 2010 at 06:55:33PM -0500, Serge E. Hallyn wrote:
> > Hi,
> >
> > In the section 'ACCESSING ANOTHER TASK'S CREDENTIALS', the file
> > Documentation/credentials.txt says:
> >
> > > A function need not get RCU read lock to use __task_cred() if it is holding a
> > > spinlock at the time as this implicitly holds the RCU read lock.
> >
> > AIUI, that is not actually right any more, is it? A spinlock does not
> > suffice as it does not necessarily imply an RCU read-side critical section
> > (anymore). Of course the spinlock specifically protecting updates would
> > suffice, but that's not what this is saying.
> >
> > Am I way off base?
>
> You are absolutely correct, good catch!!!
>
> Now, a spinlock still does imply an RCU read-side critical section given
> the following configuration options:
>
> o !CONFIG_PREEMPT
>
> o CONFIG_PREEMPT && CONFIG_TREE_RCU
>
> o CONFIG_PREEMPT && CONFIG_TINY_RCU
>
> However, relying on this is usually bad practice, as such code is prone
> to failure given the following configuration options:
>
> o CONFIG_PREEMPT && CONFIG_TREE_PREEMPT_RCU
>
> o CONFIG_PREEMPT_RT (given the -rt patchset)
>
> And when I get my act together and complete CONFIG_TINY_PREEMPT_RCU,
> then CONFIG_PREEMPT && CONFIG_TINY_PREEMPT_RCU will also invalidate
> the assumption that holding a spinlock acts as an RCU read-side
> critical section.
>
> Did you want to submit a patch for this?

Yup, sent, thanks.

-serge


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