----- 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. 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
