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

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Oct 03, 2019 at 12:35:30PM -0400, Mathieu Desnoyers wrote:
> ----- On Oct 2, 2019, at 9:43 PM, paulmck paulmck@kernel.org wrote:
> 
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > Although the rcu_swap_protected() macro follows the example of swap(),
> > the interactions with RCU make its update of its argument somewhat
> > counter-intuitive.  This commit therefore introduces an rcu_replace()
> > that returns the old value of the RCU pointer instead of doing the
> > argument update.  Once all the uses of rcu_swap_protected() are updated
> > to instead use rcu_replace(), rcu_swap_protected() will be removed.
> 
> We expose the rcu_xchg_pointer() API in liburcu (Userspace RCU) project.
> Any reason for not going that way and keep the kernel and user-space RCU
> APIs alike ?
> 
> It's of course fine if they diverge, but we might want to at least consider
> if using the same API name would be OK.

Different semantics.  An rcu_xchg_pointer() allows concurrent updates,
and rcu_replace() does not.

But yes, if someone needs the concurrent updates, rcu_xchg_pointer()
would certainly be my choice for the name.

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> 
> > 
> > Link:
> > https://lore.kernel.org/lkml/CAHk-=wiAsJLw1egFEE=Z7-GGtM6wcvtyytXZA1+BHqta4gg6Hw@mail.gmail.com/
> > Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Bart Van Assche <bart.vanassche@wdc.com>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Hannes Reinecke <hare@suse.de>
> > Cc: Johannes Thumshirn <jthumshirn@suse.de>
> > Cc: Shane M Seymour <shane.seymour@hpe.com>
> > Cc: Martin K. Petersen <martin.petersen@oracle.com>
> > ---
> > include/linux/rcupdate.h | 18 ++++++++++++++++++
> > 1 file changed, 18 insertions(+)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 75a2ede..3b73287 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -383,6 +383,24 @@ do {									      \
> > } while (0)
> > 
> > /**
> > + * rcu_replace() - replace an RCU pointer, returning its old value
> > + * @rcu_ptr: RCU pointer, whose old value is returned
> > + * @ptr: regular pointer
> > + * @c: the lockdep conditions under which the dereference will take place
> > + *
> > + * Perform a replacement, where @rcu_ptr is an RCU-annotated
> > + * pointer and @c is the lockdep argument that is passed to the
> > + * rcu_dereference_protected() call used to read that pointer.  The old
> > + * value of @rcu_ptr is returned, and @rcu_ptr is set to @ptr.
> > + */
> > +#define rcu_replace(rcu_ptr, ptr, c)					\
> > +({									\
> > +	typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));	\
> > +	rcu_assign_pointer((rcu_ptr), (ptr));				\
> > +	__tmp;								\
> > +})
> > +
> > +/**
> >  * rcu_swap_protected() - swap an RCU and a regular pointer
> >  * @rcu_ptr: RCU pointer
> >  * @ptr: regular pointer
> > --
> > 2.9.5
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com