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

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Date:   Thu, 3 Oct 2019 12:31:38 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     David Howells <dhowells@...hat.com>, paulmck@...nel.org,
        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>,
        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 K . Petersen" <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 9:08 AM, rostedt rostedt@...dmis.org wrote:

> On Thu, 03 Oct 2019 09:39:17 +0100
> David Howells <dhowells@...hat.com> wrote:
> 
>> paulmck@...nel.org wrote:
>> 
>> > +#define rcu_replace(rcu_ptr, ptr, c)					\
>> > +({									\
>> > +	typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));	\
>> > +	rcu_assign_pointer((rcu_ptr), (ptr));				\
>> > +	__tmp;								\
>> > +})
>> 
>> Does it make sense to actually use xchg() if that's supported by the arch?
>> 
> 
> Hmm, is there really any arch that doesn't support xchg()? It would be
> very hard to do any kind of atomic operations without it.
> 
> cmpxchg() is the one that I understand is optional by the arch.

I remember going through all kernel arch headers myself and introduce
irq-off-based xchg and cmpxchg generic implementations for all UP
architectures lacking cmpxchg/xchg instructions.

We might want to consider simply using xchg() here.

FWIW, the API exposed by the Userspace RCU project (liburcu) for this
is:

  rcu_xchg_pointer()

Thanks,

Mathieu


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