Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752661AbbFYAQa (ORCPT ); Wed, 24 Jun 2015 20:16:30 -0400 Received: from mga03.intel.com ([134.134.136.65]:59909 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752113AbbFYAQH (ORCPT ); Wed, 24 Jun 2015 20:16:07 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.13,673,1427785200"; d="scan'208";a="513808088" Subject: [RFCv2][PATCH 3/7] fs: fsnotify: replace memory barrier in __sb_end_write() with RCU To: dave@sr71.net Cc: jack@suse.cz, viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, tim.c.chen@linux.intel.com, ak@linux.intel.com, dave.hansen@linux.intel.com From: Dave Hansen Date: Wed, 24 Jun 2015 17:16:06 -0700 References: <20150625001605.72553909@viggo.jf.intel.com> In-Reply-To: <20150625001605.72553909@viggo.jf.intel.com> Message-Id: <20150625001606.E834E08B@viggo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2481 Lines: 67 From: Dave Hansen If I sit in a loop and do write()s to small tmpfs files, __sb_end_write() is third-hottest kernel function due to its smp_mb(). __sb_end_write() uses the barrier to avoid races with freeze_super() and its calls to sb_wait_write(). But, now that freeze_super() is calling synchronize_rcu() before each sb_wait_write() call, we can use that to our advantage. The synchronize_rcu() ensures that all __sb_end_write() will see freeze_super()'s updates to s_writers.counter. That, in turn, guarantees that __sb_end_write() will try to wake up any subsequent call by freeze_super() to sb_wait_write(). Cc: Jan Kara Cc: Alexander Viro Cc: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: Paul E. McKenney Cc: Tim Chen Cc: Andi Kleen Signed-off-by: Dave Hansen --- b/fs/super.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-) diff -puN fs/super.c~selectively-do-barriers-in-__sb_end_write fs/super.c --- a/fs/super.c~selectively-do-barriers-in-__sb_end_write 2015-06-24 17:14:35.315142611 -0700 +++ b/fs/super.c 2015-06-24 17:14:35.318142745 -0700 @@ -1146,14 +1146,23 @@ out: */ void __sb_end_write(struct super_block *sb, int level) { + rcu_read_lock(); percpu_counter_dec(&sb->s_writers.counter[level-1]); /* - * Make sure s_writers are updated before we wake up waiters in - * freeze_super(). + * We are racing here with freeze_super()'s calls to + * sb_wait_write(). We want to ensure that we call + * wake_up() whenever one of those calls _might_ be + * in sb_wait_write(). + * + * Since freeze_super() does a synchronize_rcu() before + * each of its sb_wait_write() calls, it can guarantee + * that it sees our update to s_writers.counter as well + * as that we see its update to s_writers.frozen. */ - smp_mb(); - if (waitqueue_active(&sb->s_writers.wait)) + if (unlikely(sb->s_writers.frozen >= level)) wake_up(&sb->s_writers.wait); + rcu_read_unlock(); + rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_); } EXPORT_SYMBOL(__sb_end_write); _ -- 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/