Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761952AbYGOQIn (ORCPT ); Tue, 15 Jul 2008 12:08:43 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757339AbYGOQIR (ORCPT ); Tue, 15 Jul 2008 12:08:17 -0400 Received: from tomts40.bellnexxia.net ([209.226.175.97]:53296 "EHLO tomts40-srv.bellnexxia.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1760930AbYGOQIQ (ORCPT ); Tue, 15 Jul 2008 12:08:16 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AvIEABRnfEhMRKxB/2dsb2JhbACBWq1Z Date: Tue, 15 Jul 2008 12:08:13 -0400 From: Mathieu Desnoyers To: Peter Zijlstra 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 Subject: Re: [patch 01/15] Kernel Tracepoints Message-ID: <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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <1216135902.12595.214.camel@twins> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 11:56:51 up 40 days, 20:37, 5 users, load average: 1.75, 1.13, 1.24 User-Agent: Mutt/1.5.16 (2007-06-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6248 Lines: 159 * 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 ? 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/