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

From: Peter Zijlstra
Date: Tue Nov 03 2015 - 09:47:26 EST

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
> > it can only be
> > called from process context, but according to
> > 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.
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