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

[Peter Zijlstra <peterz@infradead.org>]

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.

Even call_srcu() should be IRQ-safe.