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?
Thanks
--jyh
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.)
Lai, am I missing anything here?
Thanx, Paul
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.
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