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

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Message-ID: <20190523141851.GA7523@lakrids.cambridge.arm.com>
Date:   Thu, 23 May 2019 15:19:19 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.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)'

On Thu, May 23, 2019 at 06:50:13AM -0700, Paul E. McKenney wrote:
> On Thu, May 23, 2019 at 03:32:20PM +0200, Andrea Parri wrote:
> > The expression
> > 
> >   rcu_assign_pointer(p, typeof(p) v)
> > 
> > is reported to be of type 'typeof(p)' in the documentation (c.f., e.g.,
> > Documentation/RCU/whatisRCU.txt) but this is not the case: for example,
> > the following snippet
> > 
> >   int **y;
> >   int *x;
> >   int *r0;
> > 
> >   ...
> > 
> >   r0 = rcu_assign_pointer(*y, x);
> > 
> > can currently result in the compiler warning
> > 
> >   warning: assignment to ‘int *’ from ‘uintptr_t’ {aka ‘long unsigned int’} makes pointer from integer without a cast [-Wint-conversion]
> > 
> > Cast the uintptr_t value to a typeof(p) value.
> > 
> > Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
> > Cc: "Paul E. McKenney" <paulmck@...ux.ibm.com>
> > Cc: Josh Triplett <josh@...htriplett.org>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > Cc: Lai Jiangshan <jiangshanlai@...il.com>
> > Cc: Joel Fernandes <joel@...lfernandes.org>
> > Cc: rcu@...r.kernel.org
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: Mark Rutland <mark.rutland@....com>
> > ---
> > NOTE:
> > 
> > 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,
Mark.