From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2CD8AC18E5B for ; Tue, 17 Mar 2020 15:59:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EEBAB20738 for ; Tue, 17 Mar 2020 15:59:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584460769; bh=QKH9gO8a+divkhjn6vVthIrkDYpvb66V0Iq362aPVWc=; h=Subject:From:To:Cc:Date:In-Reply-To:References:List-ID:From; b=X/TxVfpd4XxprR9D7qSUQ0lPQCO3CmtZ79t2DBivRSMk46aW6ysTsmyId47evw8ys vXkAZ1HSHm50RPsgN2s2Ry6ygAiMA9fK25WQ0/qN4dbbl0cpoz7DvcOG301kh2y1Ua KbE/QXIVvKkGLOkRJw248unRqV48Q7nPpkH7yEQQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726757AbgCQP71 (ORCPT ); Tue, 17 Mar 2020 11:59:27 -0400 Received: from mail.kernel.org ([198.145.29.99]:60416 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726294AbgCQP71 (ORCPT ); Tue, 17 Mar 2020 11:59:27 -0400 Received: from tleilax.poochiereds.net (68-20-15-154.lightspeed.rlghnc.sbcglobal.net [68.20.15.154]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E8CDA20724; Tue, 17 Mar 2020 15:59:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584460766; bh=QKH9gO8a+divkhjn6vVthIrkDYpvb66V0Iq362aPVWc=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=hVli641ctEManFaCzq3O11jVi1IXvi7M3f7R10/wM/B7rETW+kCfvBRioAEXQS1bT wUFrGuCkRGcWwHwaM+QxaArLG9Y0yu59tTDP+zag8cFE/GMWWDdyF/g8AQViXz78X5 ju/FAEeNcULe7UYCq+cjr8yLTXDFv2Y+a/Q7Lb/k= Message-ID: <46d2c16f48f1fd4ad28a85099c59ae95a9997740.camel@kernel.org> Subject: Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression From: Jeff Layton To: NeilBrown , Linus Torvalds Cc: yangerkun , kernel test robot , LKML , lkp@lists.01.org, Bruce Fields , Al Viro Date: Tue, 17 Mar 2020 11:59:24 -0400 In-Reply-To: <87k13jsyum.fsf@notabene.neil.brown.name> References: <20200308140314.GQ5972@shao2-debian> <41c83d34ae4c166f48e7969b2b71e43a0f69028d.camel@kernel.org> <923487db2c9396c79f8e8dd4f846b2b1762635c8.camel@kernel.org> <36c58a6d07b67aac751fca27a4938dc1759d9267.camel@kernel.org> <878sk7vs8q.fsf@notabene.neil.brown.name> <875zfbvrbm.fsf@notabene.neil.brown.name> <0066a9f150a55c13fcc750f6e657deae4ebdef97.camel@kernel.org> <87v9nattul.fsf@notabene.neil.brown.name> <87o8t2tc9s.fsf@notabene.neil.brown.name> <877dznu0pk.fsf@notabene.neil.brown.name> <87pndcsxc6.fsf@notabene.neil.brown.name> <87k13jsyum.fsf@notabene.neil.brown.name> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.34.4 (3.34.4-1.fc31) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2020-03-17 at 09:45 +1100, NeilBrown wrote: > > + > > + /* > > + * Tell the world we're done with it - see comment at top > > + * of this function > > This comment might be misleading. The world doesn't care. > Only this thread cares where ->fl_blocker is NULL. We need the release > semantics when some *other* thread sets fl_blocker to NULL, not when > this thread does. > I don't think we need to spell that out and I'm not against using > store_release here, but locks_delete_block cannot race with itself, so > referring to the comment at the top of this function is misleading. > > So: > Reviewed-by: NeilBrown > > but I'm not totally happy with the comments. > > Thanks Neil. We can clean up the comments before merge. How about this revision to the earlier patch? I took the liberty of poaching your your proposed verbiage: ------------------8<--------------------- >From c9fbfae0ab615e20de0bdf1ae7b27591d602f577 Mon Sep 17 00:00:00 2001 From: Jeff Layton Date: Mon, 16 Mar 2020 18:57:47 -0400 Subject: [PATCH] SQUASH: update with Neil's comments Signed-off-by: Jeff Layton --- fs/locks.c | 31 ++++++++++++++++++++++--------- 1 file changed, 22 insertions(+), 9 deletions(-) diff --git a/fs/locks.c b/fs/locks.c index eaf754ecdaa8..e74075b0e8ec 100644 --- a/fs/locks.c +++ b/fs/locks.c @@ -741,8 +741,9 @@ static void __locks_wake_up_blocks(struct file_lock *blocker) wake_up(&waiter->fl_wait); /* - * Tell the world we're done with it - see comment at - * top of locks_delete_block(). + * The setting of fl_blocker to NULL marks the official "done" + * point in deleting a block. Paired with acquire at the top + * of locks_delete_block(). */ smp_store_release(&waiter->fl_blocker, NULL); } @@ -761,11 +762,23 @@ int locks_delete_block(struct file_lock *waiter) /* * If fl_blocker is NULL, it won't be set again as this thread "owns" * the lock and is the only one that might try to claim the lock. - * Because fl_blocker is explicitly set last during a delete, it's - * safe to locklessly test to see if it's NULL. If it is, then we know - * that no new locks can be inserted into its fl_blocked_requests list, - * and we can therefore avoid doing anything further as long as that - * list is empty. + * + * We use acquire/release to manage fl_blocker so that we can + * optimize away taking the blocked_lock_lock in many cases. + * + * The smp_load_acquire guarantees two things: + * + * 1/ that fl_blocked_requests can be tested locklessly. If something + * was recently added to that list it must have been in a locked region + * *before* the locked region when fl_blocker was set to NULL. + * + * 2/ that no other thread is accessing 'waiter', so it is safe to free + * it. __locks_wake_up_blocks is careful not to touch waiter after + * fl_blocker is released. + * + * If a lockless check of fl_blocker shows it to be NULL, we know that + * no new locks can be inserted into its fl_blocked_requests list, and + * can avoid doing anything further if the list is empty. */ if (!smp_load_acquire(&waiter->fl_blocker) && list_empty(&waiter->fl_blocked_requests)) @@ -778,8 +791,8 @@ int locks_delete_block(struct file_lock *waiter) __locks_delete_block(waiter); /* - * Tell the world we're done with it - see comment at top - * of this function + * The setting of fl_blocker to NULL marks the official "done" point in + * deleting a block. Paired with acquire at the top of this function. */ smp_store_release(&waiter->fl_blocker, NULL); spin_unlock(&blocked_lock_lock); -- 2.24.1