Re: [PATCH] tools/perf, rbtree: Add RCU wrappers to make rbtree.h usable in user-space

Path	csiph.com!aioe.org!bofh.it!news.nic.it!robomod
From	Arnaldo Carvalho de Melo <acme@kernel.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] tools/perf, rbtree: Add RCU wrappers to make rbtree.h usable in user-space
Date	Sun, 05 Jul 2015 10:50:01 +0200
Message-ID	<pINMB-Ad-11@gated-at.bofh.it> (permalink)
References	<pHB18-41K-3@gated-at.bofh.it> <pHB18-41K-1@gated-at.bofh.it> <pHD2V-5G5-11@gated-at.bofh.it> <pHMIX-3df-33@gated-at.bofh.it> <pHMIX-3df-31@gated-at.bofh.it> <pHXbj-1sI-7@gated-at.bofh.it> <pI3qp-5w3-7@gated-at.bofh.it>
X-Original-To	Peter Zijlstra <peterz@infradead.org>
MIME-Version	1.0
Content-Type	text/plain; charset=us-ascii
Content-Disposition	inline
X-URL	http://acmel.wordpress.com
User-Agent	Mutt/1.5.23 (2014-03-12)
Sender	robomod@news.nic.it
List-ID	<linux-kernel.vger.kernel.org>
X-Mailing-List	linux-kernel@vger.kernel.org
Approved	robomod@news.nic.it
Lines	56
Organization	linux.* mail to news gateway
X-Original-Cc	Rusty Russell <rusty@rustcorp.com.au>, Tejun Heo <tj@kernel.org>, Jens Axboe <axboe@kernel.dk>, Jon Christopherson <jon@jons.org>, linux-kernel@vger.kernel.org
X-Original-Date	Sat, 4 Jul 2015 13:15:56 -0300
X-Original-Message-ID	<20150704161556.GD3976@kernel.org>
X-Original-References	<20150702005253.GA26440@mtj.duckdns.org> <20150702005337.GB26440@mtj.duckdns.org> <5594A9B6.2050100@jons.org> <5594AD98.4050402@jons.org> <20150702132103.GB30677@mtj.duckdns.org> <87k2uiqo5b.fsf@rustcorp.com.au> <20150703071446.GE19282@twins.programming.kicks-ass.net>
X-Original-Sender	linux-kernel-owner@vger.kernel.org
Xref	aioe.org linux.kernel:1176880

Show key headers only | View raw

Em Fri, Jul 03, 2015 at 09:14:46AM +0200, Peter Zijlstra escreveu:
> On Fri, Jul 03, 2015 at 06:21:12AM +0930, Rusty Russell wrote:
> > Looks like Peter Zijlstra is the one to take this fix...
> 
> acme is the steward of tools/perf/
> 
> > >> diff --git a/tools/perf/util/include/linux/rcupdate.h
> > >> b/tools/perf/util/include/linux/rcupdate.h
> > >> new file mode 100644
> > >> index 0000000..51c0f45
> > >> --- /dev/null
> > >> +++ b/tools/perf/util/include/linux/rcupdate.h
> > >> @@ -0,0 +1,9 @@
> > >> +#ifndef PERF_LINUX_RCUPDATE_H_
> > >> +#define PERF_LINUX_RCUPDATE_H_
> > >> +
> > >> +/* Simple trivial wrappers for now, we don't use RCU in perf user-space
> > >> (yet): */
> > >> +#define WRITE_ONCE(var, val) ((var) = (val))
> 
> It looks like perf includes linux/compiler.h so it should already have this.
> 
> > >> +#define rcu_assign_pointer(ptr, val) WRITE_ONCE(ptr, val)
> 
> That's plain wrong, WRITE_ONCE(*(ptr), (val))

Are you sure?

In the kernel, we have this sequence:

#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))

#define smp_store_release(p, v)			\
do {						\
        compiletime_assert_atomic_type(*p);	\
        smp_mb();				\
        ACCESS_ONCE(*p) = (v);			\
} while (0)


So, if you go shortcircuiting things you remove that & and that *, no?

I.e. end up with what Rusty suggested.

So, I am trying to keep as much as the semantics of the kernel not to
fall into thse traps...

Will post a RFC soon, if the rain continues preventing me from
running...

- Arnaldo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Thread

[PATCH 2/2 block/for-linus] writeback: don't drain  bdi_writeback_congested on bdi destruction Tejun Heo <tj@kernel.org> - 2015-07-02 03:00 +0200
  Re: [PATCH] tools/perf, rbtree: Add RCU wrappers to make rbtree.h  usable in user-space Tejun Heo <tj@kernel.org> - 2015-07-02 15:30 +0200
    Re: [PATCH] tools/perf, rbtree: Add RCU wrappers to make rbtree.h usable in user-space Rusty Russell <rusty@rustcorp.com.au> - 2015-07-03 02:40 +0200
      Re: [PATCH] tools/perf, rbtree: Add RCU wrappers to make rbtree.h  usable in user-space Jon Christopherson <jon@jons.org> - 2015-07-03 12:10 +0200
      Re: [PATCH] tools/perf, rbtree: Add RCU wrappers to make rbtree.h  usable in user-space Arnaldo Carvalho de Melo <acme@kernel.org> - 2015-07-05 10:50 +0200
  Re: [PATCH 2/2 block/for-linus] writeback: don't drain bdi_writeback_congested  on bdi destruction Jens Axboe <axboe@kernel.dk> - 2015-07-02 16:10 +0200

csiph-web