Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761293AbYGOQZv (ORCPT ); Tue, 15 Jul 2008 12:25:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754079AbYGOQZm (ORCPT ); Tue, 15 Jul 2008 12:25:42 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:57212 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752851AbYGOQZl (ORCPT ); Tue, 15 Jul 2008 12:25:41 -0400 Subject: Re: [patch 01/15] Kernel Tracepoints From: Peter Zijlstra To: Mathieu Desnoyers Cc: akpm@linux-foundation.org, Ingo Molnar , linux-kernel@vger.kernel.org, Masami Hiramatsu , "Frank Ch. Eigler" , Hideo AOKI , Takashi Nishiie , Steven Rostedt , Alexander Viro , Eduard - Gabriel Munteanu , Paul E McKenney In-Reply-To: <20080715160813.GB27626@Krystal> References: <20080709145929.352201601@polymtl.ca> <20080709150043.693920317@polymtl.ca> <1216108237.12595.122.camel@twins> <20080715132543.GB20037@Krystal> <1216130356.12595.184.camel@twins> <20080715142710.GC20037@Krystal> <1216132928.12595.201.camel@twins> <20080715152224.GE20037@Krystal> <1216135902.12595.214.camel@twins> <20080715160813.GB27626@Krystal> Content-Type: text/plain Date: Tue, 15 Jul 2008 18:25:49 +0200 Message-Id: <1216139149.12595.224.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.22.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5535 Lines: 142 On Tue, 2008-07-15 at 12:08 -0400, Mathieu Desnoyers wrote: > * Peter Zijlstra (peterz@infradead.org) wrote: > > On Tue, 2008-07-15 at 11:22 -0400, Mathieu Desnoyers wrote: > > > * Peter Zijlstra (peterz@infradead.org) wrote: > > > > > > > > I'm confused by the barrier games here. > > > > > > > > Why not: > > > > > > > > void **it_func; > > > > > > > > preempt_disable(); > > > > it_func = rcu_dereference((tp)->funcs); > > > > if (it_func) { > > > > for (; *it_func; it_func++) > > > > ((void(*)(proto))(*it_func))(args); > > > > } > > > > preempt_enable(); > > > > > > > > That is, why can we skip the barrier when !it_func? is that because at > > > > that time we don't actually dereference it_func and therefore cannot > > > > observe stale data? > > > > > > > > > > Exactly. I used the implementation of rcu_assign_pointer as a hint that > > > we did not need barriers when setting the pointer to NULL, and thus we > > > should not need the read barrier when reading the NULL pointer, because > > > it references no data. > > > > > > #define rcu_assign_pointer(p, v) \ > > > ({ \ > > > if (!__builtin_constant_p(v) || \ > > > ((v) != NULL)) \ > > > smp_wmb(); \ > > > (p) = (v); \ > > > }) > > > > Yeah, I saw that,.. made me wonder. It basically assumes that when we > > write: > > > > rcu_assign_pointer(foo, NULL); > > > > foo will not be used as an index or offset. > > > > I guess Paul has thought it through and verified all in-kernel use > > cases, but it still makes me feel unconfortable. > > > > > #define rcu_dereference(p) ({ \ > > > typeof(p) _________p1 = ACCESS_ONCE(p); \ > > > smp_read_barrier_depends(); \ > > > (_________p1); \ > > > }) > > > > > > But I think you are right, since we are already in unlikely code, using > > > rcu_dereference as you do is better than my use of read barrier depends. > > > It should not change anything in the assembly result except on alpha, > > > where the read_barrier_depends() is not a nop. > > > > > > I wonder if there would be a way to add this kind of NULL pointer case > > > check without overhead in rcu_dereference() on alpha. I guess not, since > > > the pointer is almost never known at compile-time. And I guess Paul must > > > already have thought about it. The only case where we could add this > > > test is when we know that we have a if (ptr != NULL) test following the > > > rcu_dereference(); we could then assume the compiler will merge the two > > > branches since they depend on the same condition. > > > > I remember seeing a thread about all this special casing NULL, but have > > never been able to find it again - my google skillz always fail me. > > > > Basically it doesn't work if you use the variable as an index/offset, > > because in that case 0 is a valid offset and you still generate a data > > dependency. > > > > IIRC the conclusion was that the gains were too small to spend more time > > on it, although I would like to hear about the special case in > > rcu_assign_pointer. > > > > /me goes use git blame.... > > > > Actually, we could probably do the following, which also adds an extra > coherency check about non-NULL pointer assumptions : > > #ifdef CONFIG_RCU_DEBUG /* this would be new */ > #define DEBUG_RCU_BUG_ON(x) BUG_ON(x) > #else > #define DEBUG_RCU_BUG_ON(x) > #endif > > #define rcu_dereference(p) ({ \ > typeof(p) _________p1 = ACCESS_ONCE(p); \ > if (p != NULL) \ > smp_read_barrier_depends(); \ > (_________p1); \ > }) > > #define rcu_dereference_non_null(p) ({ \ > typeof(p) _________p1 = ACCESS_ONCE(p); \ > DEBUG_RCU_BUG_ON(p == NULL); \ > smp_read_barrier_depends(); \ > (_________p1); \ > }) > > The use-case where rcu_dereference() would be used is when it is > followed by a null pointer check (grepping through the sources shows me > this is a very very common case). In rare cases, it is assumed that the > pointer is never NULL and it is used just after the rcu_dereference. It > those cases, the extra test could be saved on alpha by using > rcu_dereference_non_null(p), which would check the the pointer is indeed > never NULL under some debug kernel configuration. > > Does it make sense ? This would break the case where the dereferenced variable is used as an index/offset where 0 is a valid value and still generates data dependencies. So if with your new version we do: i = rcu_dereference(foo); j = table[i]; which translates into: i = ACCESS_ONCE(foo); if (i) smp_read_barrier_depends(); j = table[i]; which when i == 0, would fail to do the barrier and can thus cause j to be a wrong value. Sadly I'll have to defer to Paul to explain exactly how that can happen - I always get my head in a horrible twist with this case. -- 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/