Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761975AbYHAVO6 (ORCPT ); Fri, 1 Aug 2008 17:14:58 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760503AbYHAVKQ (ORCPT ); Fri, 1 Aug 2008 17:10:16 -0400 Received: from e1.ny.us.ibm.com ([32.97.182.141]:42948 "EHLO e1.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760906AbYHAVKO (ORCPT ); Fri, 1 Aug 2008 17:10:14 -0400 Date: Fri, 1 Aug 2008 14:10:12 -0700 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: Peter Zijlstra , 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 Subject: Re: [patch 01/15] Kernel Tracepoints Message-ID: <20080801211012.GO14851@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080715160813.GB27626@Krystal> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6730 Lines: 165 On Tue, Jul 15, 2008 at 12:08:13PM -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 big question is "why"? smp_read_barrier_depends() is pretty lightweight, after all. Thanx, Paul > 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 ? > > Mathieu > > > > > If so, does this really matter since we're already in an unlikely > > > > section? Again, if so, this deserves a comment ;-) > > > > > > > > [ still think those preempt_* calls should be called > > > > rcu_read_sched_lock() or such. ] > > > > > > > > Anyway, does this still generate better code? > > > > > > > > > > On x86_64 : > > > > > > 820: bf 01 00 00 00 mov $0x1,%edi > > > 825: e8 00 00 00 00 callq 82a > > > 82a: 48 8b 1d 00 00 00 00 mov 0x0(%rip),%rbx # 831 > > > 831: 48 85 db test %rbx,%rbx > > > 834: 75 21 jne 857 > > > 836: eb 27 jmp 85f > > > 838: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1) > > > 83f: 00 > > > 840: 48 8b 95 68 ff ff ff mov -0x98(%rbp),%rdx > > > 847: 48 8b b5 60 ff ff ff mov -0xa0(%rbp),%rsi > > > 84e: 4c 89 e7 mov %r12,%rdi > > > 851: 48 83 c3 08 add $0x8,%rbx > > > 855: ff d0 callq *%rax > > > 857: 48 8b 03 mov (%rbx),%rax > > > 85a: 48 85 c0 test %rax,%rax > > > 85d: 75 e1 jne 840 > > > 85f: bf 01 00 00 00 mov $0x1,%edi > > > 864: > > > > > > for 68 bytes. > > > > > > My original implementation was 77 bytes, so yes, we have a win. > > > > Ah, good good ! :-) > > > > -- > Mathieu Desnoyers > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- 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/