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

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Message-ID: <996944932.1292.1570130051686.JavaMail.zimbra@efficios.com>
Date:   Thu, 3 Oct 2019 15:14:11 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     paulmck <paulmck@...nel.org>
Cc:     rcu <rcu@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        dipankar <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Bart Van Assche <bart.vanassche@....com>,
        Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Shane M Seymour <shane.seymour@....com>,
        Martin <martin.petersen@...cle.com>
Subject: Re: [PATCH tip/core/rcu 1/9] rcu: Upgrade rcu_swap_protected() to
 rcu_replace()

----- On Oct 3, 2019, at 3:09 PM, paulmck paulmck@...nel.org wrote:

> On Thu, Oct 03, 2019 at 01:21:14PM -0400, Mathieu Desnoyers wrote:
>> ----- On Oct 3, 2019, at 12:52 PM, paulmck paulmck@...nel.org wrote:
>> 
>> > On Thu, Oct 03, 2019 at 12:35:30PM -0400, Mathieu Desnoyers wrote:
>> >> ----- On Oct 2, 2019, at 9:43 PM, paulmck paulmck@...nel.org wrote:
>> >> 
>> >> > From: "Paul E. McKenney" <paulmck@...nel.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.
>> 
>> 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@mail.gmail.com/
>> >> > Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
>> >> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>> >> > Cc: Bart Van Assche <bart.vanassche@....com>
>> >> > Cc: Christoph Hellwig <hch@....de>
>> >> > Cc: Hannes Reinecke <hare@...e.de>
>> >> > Cc: Johannes Thumshirn <jthumshirn@...e.de>
>> >> > Cc: Shane M Seymour <shane.seymour@....com>
>> >> > Cc: Martin K. Petersen <martin.petersen@...cle.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
>> 
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
> > http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com