From patchwork Wed Oct 30 10:56:56 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Sterba X-Patchwork-Id: 11219459 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 29DCD1390 for ; Wed, 30 Oct 2019 10:56:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0704B2087E for ; Wed, 30 Oct 2019 10:56:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726882AbfJ3K4t (ORCPT ); Wed, 30 Oct 2019 06:56:49 -0400 Received: from mx2.suse.de ([195.135.220.15]:37666 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726184AbfJ3K4s (ORCPT ); Wed, 30 Oct 2019 06:56:48 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 0F092AD2B; Wed, 30 Oct 2019 10:56:47 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id BD02FDA783; Wed, 30 Oct 2019 11:56:56 +0100 (CET) From: David Sterba To: linux-btrfs@vger.kernel.org Cc: David Sterba Subject: [PATCH v2 1/5] btrfs: merge blocking_writers branches in btrfs_tree_read_lock Date: Wed, 30 Oct 2019 11:56:56 +0100 Message-Id: X-Mailer: git-send-email 2.23.0 In-Reply-To: References: MIME-Version: 1.0 Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org There are two ifs that use eb::blocking_writers. As this is a variable modified inside and outside of locks, we could minimize number of accesses to avoid problems with getting different results at different times. The access here is locked so this can only race with btrfs_tree_unlock that sets blocking_writers to 0 without lock and unsets the lock owner. The first branch is taken only if the same thread already holds the lock, the second if checks for blocking writers. Here we'd either unlock and wait, or proceed. Both are valid states of the locking protocol. Signed-off-by: David Sterba Reviewed-by: Johannnes Thumshirn --- fs/btrfs/locking.c | 27 ++++++++++++++------------- 1 file changed, 14 insertions(+), 13 deletions(-) diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c index 93146b495276..c84c650e56c7 100644 --- a/fs/btrfs/locking.c +++ b/fs/btrfs/locking.c @@ -128,20 +128,21 @@ void btrfs_tree_read_lock(struct extent_buffer *eb) read_lock(&eb->lock); BUG_ON(eb->blocking_writers == 0 && current->pid == eb->lock_owner); - if (eb->blocking_writers && current->pid == eb->lock_owner) { - /* - * This extent is already write-locked by our thread. We allow - * an additional read lock to be added because it's for the same - * thread. btrfs_find_all_roots() depends on this as it may be - * called on a partly (write-)locked tree. - */ - BUG_ON(eb->lock_nested); - eb->lock_nested = true; - read_unlock(&eb->lock); - trace_btrfs_tree_read_lock(eb, start_ns); - return; - } if (eb->blocking_writers) { + if (current->pid == eb->lock_owner) { + /* + * This extent is already write-locked by our thread. + * We allow an additional read lock to be added because + * it's for the same thread. btrfs_find_all_roots() + * depends on this as it may be called on a partly + * (write-)locked tree. + */ + BUG_ON(eb->lock_nested); + eb->lock_nested = true; + read_unlock(&eb->lock); + trace_btrfs_tree_read_lock(eb, start_ns); + return; + } read_unlock(&eb->lock); wait_event(eb->write_lock_wq, eb->blocking_writers == 0);