Re: Documentation/credentials.txt

["Serge E. Hallyn" <serue@us.ibm.com>]

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