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

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: paulmck <paulmck@kernel.org>
Cc: rcu <rcu@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	dipankar <dipankar@in.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	rostedt <rostedt@goodmis.org>,
	David Howells <dhowells@redhat.com>,
	Eric Dumazet <edumazet@google.com>, fweisbec <fweisbec@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>,
	"Joel Fernandes, Google" <joel@joelfernandes.org>,
	Bart Van Assche <bart.vanassche@wdc.com>,
	Christoph Hellwig <hch@lst.de>, Hannes Reinecke <hare@suse.de>,
	Johannes Thumshirn <jthumshirn@suse.de>,
	Shane M Seymour <shane.seymour@hpe.com>,
	Martin <martin.petersen@oracle.com>
Subject: Re: [PATCH tip/core/rcu 1/9] rcu: Upgrade rcu_swap_protected() to rcu_replace()
Date: Thu, 3 Oct 2019 13:21:14 -0400 (EDT)	[thread overview]
Message-ID: <618064891.1049.1570123274780.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20191003165220.GZ2689@paulmck-ThinkPad-P72>

----- On Oct 3, 2019, at 12:52 PM, paulmck paulmck@kernel.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@kernel.org wrote:
>> 
>> > From: "Paul E. McKenney" <paulmck@kernel.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 ?

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@linux-foundation.org>
>> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>> > Cc: Bart Van Assche <bart.vanassche@wdc.com>
>> > Cc: Christoph Hellwig <hch@lst.de>
>> > Cc: Hannes Reinecke <hare@suse.de>
>> > Cc: Johannes Thumshirn <jthumshirn@suse.de>
>> > Cc: Shane M Seymour <shane.seymour@hpe.com>
>> > Cc: Martin K. Petersen <martin.petersen@oracle.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