On Thu, Oct 03, 2019 at 01:21:14PM -0400, Mathieu Desnoyers wrote: > ----- On Oct 3, 2019, at 12:52 PM, paulmck paulmck@xxxxxxxxxx wrote: > > > 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. > > Then considering that its assignment counterpart is "rcu_assign_pointer()" > (and not "rcu_assign()"), would "rcu_replace_pointer()" be less ambiguous > about its intended use ? The sequence was rcu_swap_protected() -> rcu_swap() -> rcu_replace(). Because that rcu_replace(), unlike rcu_swap_protected(), returns a value, the shorter name is valuable. Maybe we should have used rcu_assign() instead of rcu_assign_pointer(), but there is no point in that sort of change at this late date! Thanx, Paul > Thanks, > > Mathieu > > > > > > 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 > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com
