----- On Oct 3, 2019, at 3:09 PM, paulmck paulmck@xxxxxxxxxx wrote: > 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! I agree that having both the "rcu_" prefix and the "_pointer" suffix is somewhat redundant. Indeed it's not a worthwhile change for a pre-existing API, but it is welcome for a new API. Thanks! Mathieu > > 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 -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
