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From: Mark Rutland <mark.rutland@arm.com>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>,
	linux-kernel@vger.kernel.org,
	Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	rcu@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will.deacon@arm.com>
Subject: Re: [RFC PATCH] rcu: Make 'rcu_assign_pointer(p, v)' of type 'typeof(p)'
Date: Thu, 23 May 2019 15:19:19 +0100	[thread overview]
Message-ID: <20190523141851.GA7523@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20190523135013.GL28207@linux.ibm.com>

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@amarulasolutions.com>
> > Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
> > Cc: Josh Triplett <josh@joshtriplett.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > Cc: Joel Fernandes <joel@joelfernandes.org>
> > Cc: rcu@vger.kernel.org
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Cc: Mark Rutland <mark.rutland@arm.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.

  reply	other threads:[~2019-05-23 14:19 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-23 13:32 Andrea Parri
2019-05-23 13:50 ` Paul E. McKenney
2019-05-23 14:19   ` Mark Rutland [this message]
2019-05-23 14:54     ` Andrea Parri
2019-05-23 15:23     ` Paul E. McKenney
2019-05-23 14:53   ` Andrea Parri

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