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 09:08:50 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     David Howells <dhowells@...hat.com>
Cc:     paulmck@...nel.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        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 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.

-- Steve