Re: [PATCH tip/core/rcu 1/9] rcu: Upgrade rcu_swap_protected() to rcu_replace()

[Mathieu Desnoyers]

----- On Oct 3, 2019, at 9:08 AM, rostedt rostedt@xxxxxxxxxxx wrote:

> On Thu, 03 Oct 2019 09:39:17 +0100
> David Howells <dhowells@xxxxxxxxxx> wrote:
> 
>> paulmck@xxxxxxxxxx wrote:
>> 
>> > +#define rcu_replace(rcu_ptr, ptr, c)					\
>> > +({									\
>> > +	typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));	\
>> > +	rcu_assign_pointer((rcu_ptr), (ptr));				\
>> > +	__tmp;								\
>> > +})
>> 
>> Does it make sense to actually use xchg() if that's supported by the arch?
>> 
> 
> Hmm, is there really any arch that doesn't support xchg()? It would be
> very hard to do any kind of atomic operations without it.
> 
> cmpxchg() is the one that I understand is optional by the arch.

I remember going through all kernel arch headers myself and introduce
irq-off-based xchg and cmpxchg generic implementations for all UP
architectures lacking cmpxchg/xchg instructions.

We might want to consider simply using xchg() here.

FWIW, the API exposed by the Userspace RCU project (liburcu) for this
is:

  rcu_xchg_pointer()

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com