From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753631Ab2KKS1R (ORCPT ); Sun, 11 Nov 2012 13:27:17 -0500 Received: from mx1.redhat.com ([209.132.183.28]:21089 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753593Ab2KKS1P (ORCPT ); Sun, 11 Nov 2012 13:27:15 -0500 Date: Sun, 11 Nov 2012 19:27:44 +0100 From: Oleg Nesterov To: Andrew Morton Cc: Linus Torvalds , "Paul E. McKenney" , Mikulas Patocka , Peter Zijlstra , Ingo Molnar , Srikar Dronamraju , Ananth N Mavinakayanahalli , Anton Arapov , linux-kernel@vger.kernel.org Subject: [PATCH -mm] percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessari ly.fix Message-ID: <20121111182744.GA25282@redhat.com> References: <20121019192838.GM2518@linux.vnet.ibm.com> <20121030184800.GA16129@redhat.com> <20121031194135.GA504@redhat.com> <20121031194158.GB504@redhat.com> <20121102180606.GA13255@redhat.com> <20121108134805.GA23870@redhat.com> <20121108134849.GB23870@redhat.com> <20121108120700.42d438f2.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121108120700.42d438f2.akpm@linux-foundation.org> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org More include's and more comments, no changes in code. To remind, once/if I am sure you agree with this patch I'll send 2 additional and simple patches: 1. lockdep annotations 2. CONFIG_PERCPU_RWSEM It seems that we can do much more improvements to a) speedup the writers and b) make percpu_rw_semaphore more useful, but not right now. Signed-off-by: Oleg Nesterov --- lib/percpu-rwsem.c | 35 +++++++++++++++++++++++++++++++++-- 1 files changed, 33 insertions(+), 2 deletions(-) diff --git a/lib/percpu-rwsem.c b/lib/percpu-rwsem.c index 0e3bc0f..02bd157 100644 --- a/lib/percpu-rwsem.c +++ b/lib/percpu-rwsem.c @@ -1,6 +1,11 @@ +#include +#include +#include +#include #include #include #include +#include int percpu_init_rwsem(struct percpu_rw_semaphore *brw) { @@ -21,6 +26,29 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw) brw->fast_read_ctr = NULL; /* catch use after free bugs */ } +/* + * This is the fast-path for down_read/up_read, it only needs to ensure + * there is no pending writer (!mutex_is_locked() check) and inc/dec the + * fast per-cpu counter. The writer uses synchronize_sched() to serialize + * with the preempt-disabled section below. + * + * The nontrivial part is that we should guarantee acquire/release semantics + * in case when + * + * R_W: down_write() comes after up_read(), the writer should see all + * changes done by the reader + * or + * W_R: down_read() comes after up_write(), the reader should see all + * changes done by the writer + * + * If this helper fails the callers rely on the normal rw_semaphore and + * atomic_dec_and_test(), so in this case we have the necessary barriers. + * + * But if it succeeds we do not have any barriers, mutex_is_locked() or + * __this_cpu_add() below can be reordered with any LOAD/STORE done by the + * reader inside the critical section. See the comments in down_write and + * up_write below. + */ static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val) { bool success = false; @@ -98,6 +126,7 @@ void percpu_down_write(struct percpu_rw_semaphore *brw) * * 3. Ensures that if any reader has exited its critical section via * fast-path, it executes a full memory barrier before we return. + * See R_W case in the comment above update_fast_ctr(). */ synchronize_sched(); @@ -116,8 +145,10 @@ void percpu_up_write(struct percpu_rw_semaphore *brw) /* allow the new readers, but only the slow-path */ up_write(&brw->rw_sem); - /* insert the barrier before the next fast-path in down_read */ + /* + * Insert the barrier before the next fast-path in down_read, + * see W_R case in the comment above update_fast_ctr(). + */ synchronize_sched(); - mutex_unlock(&brw->writer_mutex); } -- 1.5.5.1