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=-15.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 4DA63C433ED for ; Fri, 16 Apr 2021 21:25:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 33AB56023F for ; Fri, 16 Apr 2021 21:25:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344341AbhDPVZ3 (ORCPT ); Fri, 16 Apr 2021 17:25:29 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:24786 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236093AbhDPVZ2 (ORCPT ); Fri, 16 Apr 2021 17:25:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1618608302; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=TKRy1IG7OPwL6hb/OAIIdkCcfyxk1VBZfq/Tnz82kWE=; b=UaTHyylkdEnjASiceaE+4LT6C5l5pxXpvVDX4kwCOuYoXO3BRcVkaxATurCUQztxRCnaFx YIwv+6s/G9lJc23Nz5AJcrRMljN61tZ5wLUTyjUYDsSqX/jcaQgk5ODbvRqgU7kxJslF2c IMDjCyPBa2xl9/AF73QmD8Q7BPyF6FM= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-16-MziAUdMdON65qhCCP-cy0A-1; Fri, 16 Apr 2021 17:25:00 -0400 X-MC-Unique: MziAUdMdON65qhCCP-cy0A-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id BD4256D582; Fri, 16 Apr 2021 21:24:59 +0000 (UTC) Received: from horse.redhat.com (ovpn-116-117.rdu2.redhat.com [10.10.116.117]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6E359107D5C3; Fri, 16 Apr 2021 21:24:50 +0000 (UTC) Received: by horse.redhat.com (Postfix, from userid 10451) id 045FF22054F; Fri, 16 Apr 2021 17:24:49 -0400 (EDT) Date: Fri, 16 Apr 2021 17:24:49 -0400 From: Vivek Goyal To: Dan Williams Cc: Linux fsdevel mailing list , Jan Kara , Matthew Wilcox , virtio-fs-list , Sergio Lopez , Miklos Szeredi , linux-nvdimm , linux kernel mailing list Subject: Re: [PATCH] dax: Fix missed wakeup in put_unlocked_entry() Message-ID: <20210416212449.GB1379987@redhat.com> References: <20210416173524.GA1379987@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 16, 2021 at 12:56:05PM -0700, Dan Williams wrote: > On Fri, Apr 16, 2021 at 10:35 AM Vivek Goyal wrote: > > > > I am seeing missed wakeups which ultimately lead to a deadlock when I am > > using virtiofs with DAX enabled and running "make -j". I had to mount > > virtiofs as rootfs and also reduce to dax window size to 32M to reproduce > > the problem consistently. > > > > This is not a complete patch. I am just proposing this partial fix to > > highlight the issue and trying to figure out how it should be fixed. > > Should it be fixed in generic dax code or should filesystem (fuse/virtiofs) > > take care of this. > > > > So here is the problem. put_unlocked_entry() wakes up waiters only > > if entry is not null as well as !dax_is_conflict(entry). But if I > > call multiple instances of invalidate_inode_pages2() in parallel, > > then I can run into a situation where there are waiters on > > this index but nobody will wait these. > > > > invalidate_inode_pages2() > > invalidate_inode_pages2_range() > > invalidate_exceptional_entry2() > > dax_invalidate_mapping_entry_sync() > > __dax_invalidate_entry() { > > xas_lock_irq(&xas); > > entry = get_unlocked_entry(&xas, 0); > > ... > > ... > > dax_disassociate_entry(entry, mapping, trunc); > > xas_store(&xas, NULL); > > ... > > ... > > put_unlocked_entry(&xas, entry); > > xas_unlock_irq(&xas); > > } > > > > Say a fault in in progress and it has locked entry at offset say "0x1c". > > Now say three instances of invalidate_inode_pages2() are in progress > > (A, B, C) and they all try to invalidate entry at offset "0x1c". Given > > dax entry is locked, all tree instances A, B, C will wait in wait queue. > > > > When dax fault finishes, say A is woken up. It will store NULL entry > > at index "0x1c" and wake up B. When B comes along it will find "entry=0" > > at page offset 0x1c and it will call put_unlocked_entry(&xas, 0). And > > this means put_unlocked_entry() will not wake up next waiter, given > > the current code. And that means C continues to wait and is not woken > > up. > > > > In my case I am seeing that dax page fault path itself is waiting > > on grab_mapping_entry() and also invalidate_inode_page2() is > > waiting in get_unlocked_entry() but entry has already been cleaned > > up and nobody woke up these processes. Atleast I think that's what > > is happening. > > > > This patch wakes up a process even if entry=0. And deadlock does not > > happen. I am running into some OOM issues, that will debug. > > > > So my question is that is it a dax issue and should it be fixed in > > dax layer. Or should it be handled in fuse to make sure that > > multiple instances of invalidate_inode_pages2() on same inode > > don't make progress in parallel and introduce enough locking > > around it. > > > > Right now fuse_finish_open() calls invalidate_inode_pages2() without > > any locking. That allows it to make progress in parallel to dax > > fault path as well as allows multiple instances of invalidate_inode_pages2() > > to run in parallel. > > > > Not-yet-signed-off-by: Vivek Goyal > > --- > > fs/dax.c | 7 ++++--- > > 1 file changed, 4 insertions(+), 3 deletions(-) > > > > Index: redhat-linux/fs/dax.c > > =================================================================== > > --- redhat-linux.orig/fs/dax.c 2021-04-16 12:50:40.141363317 -0400 > > +++ redhat-linux/fs/dax.c 2021-04-16 12:51:42.385926390 -0400 > > @@ -266,9 +266,10 @@ static void wait_entry_unlocked(struct x > > > > static void put_unlocked_entry(struct xa_state *xas, void *entry) > > { > > - /* If we were the only waiter woken, wake the next one */ > > - if (entry && !dax_is_conflict(entry)) > > - dax_wake_entry(xas, entry, false); > > + if (dax_is_conflict(entry)) > > + return; > > + > > + dax_wake_entry(xas, entry, false); > Hi Dan, > How does this work if entry is NULL? dax_entry_waitqueue() will not > know if it needs to adjust the index. Wake waiters both at current index as well PMD adjusted index. It feels little ugly though. > I think the fix might be to > specify that put_unlocked_entry() in the invalidate path needs to do a > wake_up_all(). Doing a wake_up_all() when we invalidate an entry, sounds good. I will give it a try. Thanks Vivek