Re: Can srcu_read_lock() be called from interrupt context?

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Nov 03, 2015 at 03:47:17PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 03, 2015 at 01:55:39AM -0800, Paul E. McKenney wrote:
> > On Mon, Nov 02, 2015 at 05:56:56PM -0800, Yunhong Jiang wrote:
> > > Hi, Paul
> > > 	I have a question to the srcu_read_lock(). Can it be invoked on 
> > > interrupt context? According to 
> > > http://lxr.free-electrons.com/source/kernel/rcu/srcu.c#L292 it can only be 
> > > called from process context, but according to 
> > > http://lxr.free-electrons.com/source/include/linux/srcu.h#L213 seems it's ok 
> > > from irq context only if it matches with the srcu_read_unlock. Can you 
> > > please give some hints?
> > 
> > Adding Lai Jianshan for his thoughts.
> > 
> > I believe that srcu.h is correct, at least assuming that interrupts do
> > not nest too deeply.  (If they were to nest four billion deep, then the
> > ->seq[] counter could overflow, defeating the checks, but the CPU stack
> > would have overflowed long before.)
> 
> It seems like a strange constraint to me; not being able to use
> srcu_read_lock() from IRQ (or even NMI) context. And looking at the
> various implementations of it nothing ever prohibited this.
> 
> While srcu _allows_ for sleeping while holding the read side primitives,
> it is not required at all.
> 
> So I would suggest amending the comment and RCU/checklist.txt.

We need to hear from Lai.  I -think- that it is OK for one srcu_read_lock()
to interrupt another, but I could easily be missing something.

> Even call_srcu() should be IRQ-safe.

Looks plausible to me.  Lai?

							Thanx, Paul