Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752897AbbHURqI (ORCPT ); Fri, 21 Aug 2015 13:46:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54829 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751807AbbHURpZ (ORCPT ); Fri, 21 Aug 2015 13:45:25 -0400 Date: Fri, 21 Aug 2015 19:43:00 +0200 From: Oleg Nesterov To: Paul McKenney Cc: Ingo Molnar , Linus Torvalds , Peter Zijlstra , Tejun Heo , linux-kernel@vger.kernel.org Subject: [PATCH v2 7/8] percpu-rwsem: fix the comments outdated by rcu_sync Message-ID: <20150821174300.GA17913@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150821174230.GA17867@redhat.com> 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 Content-Length: 4035 Lines: 97 Update the comments broken by the previous change. Signed-off-by: Oleg Nesterov --- kernel/locking/percpu-rwsem.c | 50 +++++++++-------------------------------- 1 files changed, 11 insertions(+), 39 deletions(-) diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c index d3103da..69704eb 100644 --- a/kernel/locking/percpu-rwsem.c +++ b/kernel/locking/percpu-rwsem.c @@ -38,27 +38,12 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw) } /* - * This is the fast-path for down_read/up_read, it only needs to ensure - * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the - * fast per-cpu counter. The writer uses synchronize_sched_expedited() 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 + * This is the fast-path for down_read/up_read. If it succeeds we rely + * on the barriers provided by rcu_sync_enter/exit; see the comments in + * percpu_down_write() and percpu_up_write(). * * 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, atomic_read(write_ctr) 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) { @@ -133,29 +118,15 @@ static int clear_fast_ctr(struct percpu_rw_semaphore *brw) return sum; } -/* - * A writer increments ->write_ctr to force the readers to switch to the - * slow mode, note the atomic_read() check in update_fast_ctr(). - * - * After that the readers can only inc/dec the slow ->slow_read_ctr counter, - * ->fast_read_ctr is stable. Once the writer moves its sum into the slow - * counter it represents the number of active readers. - * - * Finally the writer takes ->rw_sem for writing and blocks the new readers, - * then waits until the slow counter becomes zero. - */ void percpu_down_write(struct percpu_rw_semaphore *brw) { /* - * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read - * so that update_fast_ctr() can't succeed. - * - * 2. Ensures we see the result of every previous this_cpu_add() in - * update_fast_ctr(). + * Make rcu_sync_is_idle() == F and thus disable the fast-path in + * percpu_down_read() and percpu_up_read(), and wait for gp pass. * - * 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(). + * The latter synchronises us with the preceeding readers which used + * the fast-past, so we can not miss the result of __this_cpu_add() + * or anything else inside their criticial sections. */ rcu_sync_enter(&brw->rss); @@ -174,8 +145,9 @@ void percpu_up_write(struct percpu_rw_semaphore *brw) /* release the lock, but the readers can't use the fast-path */ up_write(&brw->rw_sem); /* - * Insert the barrier before the next fast-path in down_read, - * see W_R case in the comment above update_fast_ctr(). + * Enable the fast-path in percpu_down_read() and percpu_up_read() + * but only after another gp pass; this adds the necessary barrier + * to ensure the reader can't miss the changes done by us. */ rcu_sync_exit(&brw->rss); } -- 1.5.5.1 -- 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/