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From: Joel Fernandes <joel@joelfernandes.org>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	rcu <rcu@vger.kernel.org>, Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH RFC v2] rcu/segcblist: Add counters to segcblist datastructure
Date: Mon, 27 Jul 2020 19:03:23 -0400
Message-ID: <20200727230323.GA3390623@google.com> (raw)
In-Reply-To: <20200727134925.GB79404@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net>

On Mon, Jul 27, 2020 at 09:49:25PM +0800, Boqun Feng wrote:
[...] 
> So C11 features are available to gnu89 as extensions, also I tried to
> compile the following code with -std=gnu89:
> 
> 	#include <stdio.h>
> 
> 	typedef struct {
> 		int a;
> 	} atomic_t;
> 
> 	void g(void) {
> 		printf("this is g\n");
> 	}
> 
> 	void h(void) {
> 		printf("this is f\n");
> 	}
> 
> 	#define gen(x) _Generic((x), atomic_t : h(), int : g())
> 
> 	int main(void) {
> 		int a;
> 		atomic_t b;
> 		gen(a);
> 		gen(b);
> 		gen(b);
> 	}
> 
> , and it worked.
> 
> Besides, please note that in include/linux/compiler_types.h, _Generic is
> already used.

> > > With these primitives introduced, you can avoid () to add those
> > > rcu_segcblist_*_seglen() which have #ifdefs in them. Of course, an
> > > alternative would be that we implement rcu_segcblist_*_seglen() using
> > > _Generic, but I think someone else may have the similar problems or
> > > requirement (already or in the future), so it might be worthwhile to
> > > introduce the gen_ primitives for broader usage.
> > 
> > One issue is code using memory barriers around the operation, such as
> > in rcu_segcblist_add_len() where you use smp_mb__before_atomic() for
> > the atomic version, and regular smp_mb() for the non-atomic version.
> > So ifdef will still exist to some degree.
> > 
> 
> Right, I think we can have two functions: long_add_mb() and
> atomic_long_add_mb(), this part is similar to ifdef approach, but we can
> make a gen_long_add_mb() based on these two functions, and
> gen_long_add_mb() simply switches between those functions according to
> the actual type of the field, which I think is better than ifdef
> approach at readability and maintenance.

Thanks for clarification. I agree with your idea, would you be able to write
a patch to add the helpers my patch can use?

If others are Ok with your idea, I can go ahead and use your helpers.

(I could write the helpers myself as well, next time I send the patch).

thanks,

 - Joel


  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-19  3:55 Joel Fernandes (Google)
2020-07-19  4:06 ` Joel Fernandes
2020-07-19  4:18   ` Joel Fernandes
2020-07-20  8:22     ` boqun.feng
2020-07-24 19:34       ` Joel Fernandes
2020-07-27 13:49         ` Boqun Feng
2020-07-27 23:03           ` Joel Fernandes [this message]
2020-07-29  1:28             ` Boqun Feng

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