On Thu, Oct 03, 2019 at 12:35:30PM -0400, Mathieu Desnoyers wrote: > ----- On Oct 2, 2019, at 9:43 PM, paulmck paulmck@xxxxxxxxxx wrote: > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > > > 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@xxxxxxxxxxxxxx/ > > Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > Cc: Bart Van Assche <bart.vanassche@xxxxxxx> > > Cc: Christoph Hellwig <hch@xxxxxx> > > Cc: Hannes Reinecke <hare@xxxxxxx> > > Cc: Johannes Thumshirn <jthumshirn@xxxxxxx> > > Cc: Shane M Seymour <shane.seymour@xxxxxxx> > > Cc: Martin K. Petersen <martin.petersen@xxxxxxxxxx> > > --- > > 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
