linux-kernel - Re: [RFC PATCH] rcu: Make 'rcu_assign_pointer(p, v)' of type 'typeof(p)'

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Message-ID: <20190523145434.GB18692@andrea>
Date:   Thu, 23 May 2019 16:54:34 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [RFC PATCH] rcu: Make 'rcu_assign_pointer(p, v)' of type
 'typeof(p)'

> > > TBH, I'm not sure this is 'the right patch' (hence the RFC...): in
> > > fact, I'm currently missing the motivations for allowing assignments
> > > such as the "r0 = ..." assignment above in generic code.  (BTW, it's
> > > not currently possible to use such assignments in litmus tests...)
> > 
> > Given that a quick (and perhaps error-prone) search of the uses of
> > rcu_assign_pointer() in v5.1 didn't find a single use of the return
> > value, let's please instead change the documentation and implementation
> > to eliminate the return value.
> 
> FWIW, I completely agree, and for similar reasons I'd say we should do
> the same to WRITE_ONCE(), where this 'cool feature' has been inherited
> from.
> 
> For WRITE_ONCE() there's at least one user that needs to be cleaned up
> first (relying on non-portable implementation detaisl of atomic*_set()),
> but I suspect rcu_assign_pointer() isn't used as much as a building
> block for low-level macros.

Thanks for the confirmation, Mark.

I can look at the WRITE_ONCE() issues (user and implementation); it will
probably be a separate patchset...

Thanks,
  Andrea