Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752532Ab2B0H4h (ORCPT ); Mon, 27 Feb 2012 02:56:37 -0500 Received: from cn.fujitsu.com ([222.73.24.84]:63599 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1752228Ab2B0H4f (ORCPT ); Mon, 27 Feb 2012 02:56:35 -0500 Message-ID: <4F4B3840.6000504@cn.fujitsu.com> Date: Mon, 27 Feb 2012 16:01:04 +0800 From: Lai Jiangshan User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc14 Thunderbird/3.1.4 MIME-Version: 1.0 To: paulmck@linux.vnet.ibm.com CC: linux-kernel@vger.kernel.org, mingo@elte.hu, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, patches@linaro.org Subject: [PATCH 2/2 RFC] srcu: implement Peter's checking algorithm References: <20120213020951.GA12138@linux.vnet.ibm.com> <4F41F315.1040900@cn.fujitsu.com> <20120220174418.GI2470@linux.vnet.ibm.com> <4F42EF53.6060400@cn.fujitsu.com> <20120221015037.GE2384@linux.vnet.ibm.com> <4F435966.9020106@cn.fujitsu.com> <20120221172442.GG2375@linux.vnet.ibm.com> <4F44B580.6040003@cn.fujitsu.com> <4F4744E9.1060109@cn.fujitsu.com> <20120224200109.GH2399@linux.vnet.ibm.com> In-Reply-To: <20120224200109.GH2399@linux.vnet.ibm.com> X-MIMETrack: Itemize by SMTP Server on mailserver/fnst(Release 8.5.1FP4|July 25, 2010) at 2012-02-27 15:54:41, Serialize by Router on mailserver/fnst(Release 8.5.1FP4|July 25, 2010) at 2012-02-27 15:54:47, Serialize complete at 2012-02-27 15:54:47 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9933 Lines: 261 >From 40724998e2d121c2b5a5bd75114625cfd9d4f9a9 Mon Sep 17 00:00:00 2001 From: Lai Jiangshan Date: Mon, 27 Feb 2012 14:22:47 +0800 Subject: [PATCH 2/2] srcu: implement Peter's checking algorithm This patch implement the algorithm as Peter's: https://lkml.org/lkml/2012/2/1/119 o Make the checking lock-free and we can perform parallel checking, Although almost parallel checking makes no sense, but we need it when 1) the original checking task is preempted for long, 2) sychronize_srcu_expedited(), 3) avoid lock(see next) o Since it is lock-free, we save a mutex in state machine for call_srcu(). o Remove the SRCU_REF_MASK and remove the coupling with the flipping. (so we can remove the preempt_disable() in future, but use __this_cpu_inc() instead.) o reduce a smp_mb(), simplify the comments and make the smp_mb() pairs more intuitive. Inspired-by: Peter Zijlstra Signed-off-by: Lai Jiangshan --- include/linux/srcu.h | 7 +-- kernel/srcu.c | 137 ++++++++++++++++++++----------------------------- 2 files changed, 57 insertions(+), 87 deletions(-) diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 5b49d41..15354db 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -32,18 +32,13 @@ struct srcu_struct_array { unsigned long c[2]; + unsigned long seq[2]; }; -/* Bit definitions for field ->c above and ->snap below. */ -#define SRCU_USAGE_BITS 1 -#define SRCU_REF_MASK (ULONG_MAX >> SRCU_USAGE_BITS) -#define SRCU_USAGE_COUNT (SRCU_REF_MASK + 1) - struct srcu_struct { unsigned completed; struct srcu_struct_array __percpu *per_cpu_ref; struct mutex mutex; - unsigned long snap[NR_CPUS]; #ifdef CONFIG_DEBUG_LOCK_ALLOC struct lockdep_map dep_map; #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ diff --git a/kernel/srcu.c b/kernel/srcu.c index 47ee35d..376b583 100644 --- a/kernel/srcu.c +++ b/kernel/srcu.c @@ -73,10 +73,25 @@ EXPORT_SYMBOL_GPL(init_srcu_struct); #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */ /* + * Returns approximate total sequence of readers on the specified rank + * of per-CPU counters. + */ +static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx) +{ + int cpu; + unsigned long sum = 0; + unsigned long t; + + for_each_possible_cpu(cpu) { + t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]); + sum += t; + } + return sum; +} + +/* * Returns approximate number of readers active on the specified rank - * of per-CPU counters. Also snapshots each counter's value in the - * corresponding element of sp->snap[] for later use validating - * the sum. + * of per-CPU counters. */ static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx) { @@ -87,26 +102,36 @@ static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx) for_each_possible_cpu(cpu) { t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]); sum += t; - sp->snap[cpu] = t; } - return sum & SRCU_REF_MASK; + return sum; } -/* - * To be called from the update side after an index flip. Returns true - * if the modulo sum of the counters is stably zero, false if there is - * some possibility of non-zero. - */ static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx) { int cpu; + unsigned long seq; + + seq = srcu_readers_seq_idx(sp, idx); + + /* + * smp_mb() A pairs with smp_mb() B for critical section. + * It ensures that the SRCU read-side critical section whose + * read-lock is not seen by the following srcu_readers_active_idx() + * will see any updates that before the current task performed before. + * (So we don't need to care these readers this time) + * + * Also, if we see the increment of the seq, we must see the + * increment of the active counter in the following + * srcu_readers_active_idx(). + */ + smp_mb(); /* A */ /* * Note that srcu_readers_active_idx() can incorrectly return * zero even though there is a pre-existing reader throughout. * To see this, suppose that task A is in a very long SRCU * read-side critical section that started on CPU 0, and that - * no other reader exists, so that the modulo sum of the counters + * no other reader exists, so that the sum of the counters * is equal to one. Then suppose that task B starts executing * srcu_readers_active_idx(), summing up to CPU 1, and then that * task C starts reading on CPU 0, so that its increment is not @@ -122,53 +147,26 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx) return false; /* - * Since the caller recently flipped ->completed, we can see at - * most one increment of each CPU's counter from this point - * forward. The reason for this is that the reader CPU must have - * fetched the index before srcu_readers_active_idx checked - * that CPU's counter, but not yet incremented its counter. - * Its eventual counter increment will follow the read in - * srcu_readers_active_idx(), and that increment is immediately - * followed by smp_mb() B. Because smp_mb() D is between - * the ->completed flip and srcu_readers_active_idx()'s read, - * that CPU's subsequent load of ->completed must see the new - * value, and therefore increment the counter in the other rank. - */ - smp_mb(); /* A */ - - /* - * Now, we check the ->snap array that srcu_readers_active_idx() - * filled in from the per-CPU counter values. Since - * __srcu_read_lock() increments the upper bits of the per-CPU - * counter, an increment/decrement pair will change the value - * of the counter. Since there is only one possible increment, - * the only way to wrap the counter is to have a huge number of - * counter decrements, which requires a huge number of tasks and - * huge SRCU read-side critical-section nesting levels, even on - * 32-bit systems. - * - * All of the ways of confusing the readings require that the scan - * in srcu_readers_active_idx() see the read-side task's decrement, - * but not its increment. However, between that decrement and - * increment are smb_mb() B and C. Either or both of these pair - * with smp_mb() A above to ensure that the scan below will see - * the read-side tasks's increment, thus noting a difference in - * the counter values between the two passes. + * Validation step, smp_mb() D pairs with smp_mb() C. If the above + * srcu_readers_active_idx() see a decrement of the active counter + * in srcu_read_unlock(), it should see one of these for corresponding + * srcu_read_lock(): + * See the increment of the active counter, + * Failed to see the increment of the active counter. + * The second one can cause srcu_readers_active_idx() incorrectly + * return zero, but it means the above srcu_readers_seq_idx() does not + * see the increment of the seq(ref: comments of smp_mb() A), + * and the following srcu_readers_seq_idx() sees the increment of + * the seq. The seq is changed. * - * Therefore, if srcu_readers_active_idx() returned zero, and - * none of the counters changed, we know that the zero was the - * correct sum. - * - * Of course, it is possible that a task might be delayed - * for a very long time in __srcu_read_lock() after fetching - * the index but before incrementing its counter. This - * possibility will be dealt with in __synchronize_srcu(). + * This smp_mb() D pairs with smp_mb() C for critical section. + * then any of the current task's subsequent code will happen after + * that SRCU read-side critical section whose read-unlock is seen in + * srcu_readers_active_idx(). */ - for_each_possible_cpu(cpu) - if (sp->snap[cpu] != - ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx])) - return false; /* False zero reading! */ - return true; + smp_mb(); /* D */ + + return srcu_readers_seq_idx(sp, idx) == seq; } /** @@ -216,9 +214,9 @@ int __srcu_read_lock(struct srcu_struct *sp) preempt_disable(); idx = rcu_dereference_index_check(sp->completed, rcu_read_lock_sched_held()) & 0x1; - ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += - SRCU_USAGE_COUNT + 1; + ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1; smp_mb(); /* B */ /* Avoid leaking the critical section. */ + ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1; preempt_enable(); return idx; } @@ -258,17 +256,6 @@ static void wait_idx(struct srcu_struct *sp, int idx, bool expedited) int trycount = 0; /* - * If a reader fetches the index before the ->completed increment, - * but increments its counter after srcu_readers_active_idx_check() - * sums it, then smp_mb() D will pair with __srcu_read_lock()'s - * smp_mb() B to ensure that the SRCU read-side critical section - * will see any updates that the current task performed before its - * call to synchronize_srcu(), or to synchronize_srcu_expedited(), - * as the case may be. - */ - smp_mb(); /* D */ - - /* * SRCU read-side critical sections are normally short, so wait * a small amount of time before possibly blocking. */ @@ -281,18 +268,6 @@ static void wait_idx(struct srcu_struct *sp, int idx, bool expedited) schedule_timeout_interruptible(1); } } - - /* - * The following smp_mb() E pairs with srcu_read_unlock()'s - * smp_mb C to ensure that if srcu_readers_active_idx_check() - * sees srcu_read_unlock()'s counter decrement, then any - * of the current task's subsequent code will happen after - * that SRCU read-side critical section. - * - * It also ensures the order between the above waiting and - * the next flipping. - */ - smp_mb(); /* E */ } static void srcu_flip(struct srcu_struct *sp) -- 1.7.4.4 -- 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/