linux-kernel - Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of __srcu_read

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Date:	Tue, 6 Oct 2015 11:07:34 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of
 __srcu_read_lock()

On Tue, Oct 06, 2015 at 10:43:11AM -0700, Josh Triplett wrote:
> 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. :)

And they did supply an explanation that seemed satisfactory at the time,
but I cannot find that either.  I clearly need to track that sort of stuff
better!

							Thanx, Paul

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