LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: 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>,
	Mark Rutland <mark.rutland@arm.com>
Subject: Re: [RFC PATCH] rcu: Make 'rcu_assign_pointer(p, v)' of type 'typeof(p)'
Date: Thu, 23 May 2019 06:50:13 -0700	[thread overview]
Message-ID: <20190523135013.GL28207@linux.ibm.com> (raw)
In-Reply-To: <1558618340-17254-1-git-send-email-andrea.parri@amarulasolutions.com>

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.

> The usual concern is, of course, that if something is allowed (read
> 'compile!' ;/) then people will soon or later use it and they'll do
> it in all sorts of 'creative' ways, such as 'to extend dependencies
> across rcu_assign_pointer() calls' as in
> 
>   x = READ_ONCE(*z);
>   r0 = rcu_assign_pointer(*y, x);
>   WRITE_ONCE(*w, r0);
> 
> Notice that using a 'do { ... } while (0)', say, would prevent such
> tricks/rvalues. (The same approach is used by smp_store_release().)

As you in fact suggest here.  ;-)

							Thanx, Paul

> For a related discussion, please see:
> 
>   https://lkml.kernel.org/r/20190523083013.GA4616@andrea
> 
> Thoughts?
> 
>   Andrea
> ---
>  include/linux/rcupdate.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 915460ec08722..b94ba5de78fba 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -375,7 +375,7 @@ static inline void rcu_preempt_sleep_check(void) { }
>  		WRITE_ONCE((p), (typeof(p))(_r_a_p__v));		      \
>  	else								      \
>  		smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
> -	_r_a_p__v;							      \
> +	((typeof(p))_r_a_p__v);						      \
>  })
>  
>  /**
> -- 
> 2.7.4
> 

  reply	other threads:[~2019-05-23 13:52 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 [this message]
2019-05-23 14:19   ` Mark Rutland
2019-05-23 14:54     ` Andrea Parri
2019-05-23 15:23     ` Paul E. McKenney
2019-05-23 14:53   ` Andrea Parri

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190523135013.GL28207@linux.ibm.com \
    --to=paulmck@linux.ibm.com \
    --cc=andrea.parri@amarulasolutions.com \
    --cc=jiangshanlai@gmail.com \
    --cc=joel@joelfernandes.org \
    --cc=josh@joshtriplett.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mark.rutland@arm.com \
    --cc=mathieu.desnoyers@efficios.com \
    --cc=peterz@infradead.org \
    --cc=rcu@vger.kernel.org \
    --cc=rostedt@goodmis.org \
    --cc=will.deacon@arm.com \
    --subject='Re: [RFC PATCH] rcu: Make '\''rcu_assign_pointer(p, v)'\'' of type '\''typeof(p)'\''' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).