Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of __srcu_read_lock()

[Josh Triplett <josh@joshtriplett.org>]

On Tue, Oct 06, 2015 at 10:36:46AM -0700, Paul E. McKenney wrote:
> On Tue, Oct 06, 2015 at 10:18:39AM -0700, Josh Triplett wrote:
> > On Tue, Oct 06, 2015 at 09:13:42AM -0700, Paul E. McKenney wrote:
> > > Currently, __srcu_read_lock() cannot be invoked from restricted
> > > environments because it contains calls to preempt_disable() and
> > > preempt_enable(), both of which can invoke lockdep, which is a bad
> > > idea in some restricted execution modes.  This commit therefore moves
> > > the preempt_disable() and preempt_enable() from __srcu_read_lock()
> > > to srcu_read_lock().  It also inserts the preempt_disable() and
> > > preempt_enable() around the call to __srcu_read_lock() in do_exit().
> > 
> > What restricted environments do you intend to invoke
> > __srcu_read_lock from?
> > 
> > This change seems fine, but I don't see any change in this patch series
> > that needs this, hence my curiosity.
> 
> Someone asked me for it, and now I cannot find it.  :-(
> 
> Something to the effect of when running unmapped during exception entry
> or something like that.  I guess one way to find out would be to remove
> the commit and see who complained, but on the other hand, it arguably
> makes more sense to have only the bare mechanism is __srcu_read_lock()
> and put the environmental protection into srcu_read_lock().

I agree; I just find the idea that someone would need to call
__srcu_read_lock rather than srcu_read_lock odd and worthy of further
understanding. :)