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=-13.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,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 D9285C433ED for ; Mon, 19 Apr 2021 19:49:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 942B961077 for ; Mon, 19 Apr 2021 19:49:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240696AbhDSTtk (ORCPT ); Mon, 19 Apr 2021 15:49:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60748 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233647AbhDSTtj (ORCPT ); Mon, 19 Apr 2021 15:49:39 -0400 Received: from mail-ej1-x632.google.com (mail-ej1-x632.google.com [IPv6:2a00:1450:4864:20::632]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A6300C06174A for ; Mon, 19 Apr 2021 12:49:09 -0700 (PDT) Received: by mail-ej1-x632.google.com with SMTP id l4so54735829ejc.10 for ; Mon, 19 Apr 2021 12:49:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=intel-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=0pRdHORbMnYGgpAEqFhWtNH9/sxU/zrFmn1etVSS8ec=; b=pbgttAEz2va7NMNY0DPCjAIrN3cvwgh1vLPTHCTYGogu5LS30biP0lrPVzo2ndKVug c910hP3lyskv2p9CsZZmtyPWoONAJwqaunWQvnjFhjIX0Q5yZ5SChmgdBzPdhSyUJoj1 N3Sg+EygRrSverxWz+odjeJVoGWYQmzDyrbb4Xso5/JC8GcrmocAQUAI9cAzpzoQutzT 2xRp+1GLcu4i3ZpQVcRvSUUEnMYkP6bhFS1FBvK2WyY2dmofMG+PRGN1HZzKDoVhs+od o9kzI7900pFwR78M2AFerahttXmuiaLedpc5P76RshFxO1Y3+hRfMsGLOUEcZgyZvF35 hCSQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=0pRdHORbMnYGgpAEqFhWtNH9/sxU/zrFmn1etVSS8ec=; b=Lfra9dAb/ymApz4HqsdueJ+j5ZFgtnGccog34VZaNJXcZiL5I/PJAKh4zssJ8VdE7J k4+fO7kR+1TIAaqMhZHa/PAE2uwUre6OXlV6CHiIL8D7p8s0Lcsyk6qo1qq6MB0ySmA4 O94MFUJKj6ti2gXnDyX5u92CiCATpefRmDS6SSfH6mx+yNyqVarDLFZ6mJPrQOaxIa7P u4FsNkRCX+LN1Z5824RRpOGsu5ZDKrhQYrzKBs9myDqVHbK/PLbULQys18fbHjHtIJQE nxkUIzr6AfmIpsO6pHE/CyYf3Ke0AqK023RI1WSToCeYmMoRdK8H3v9K+xF9Dw9BW+VB ZcHg== X-Gm-Message-State: AOAM532hrpbtruftSuZZzVjfL6+Fe4wriPX6JZxGhmpx5FOvQ7QO4cat ftjvVzVH7/ZMIdxmQHwoppW+jYirzddSygOYmtZnzw== X-Google-Smtp-Source: ABdhPJw0kWSefwOb928/NEJy0zoj/ddLUSgM8SbRvl82iPLTA2/huvDl9OCcxhXSBS6EtSGZbyr13FXYLv8LlMcG2WM= X-Received: by 2002:a17:907:7631:: with SMTP id jy17mr23809461ejc.418.1618861748396; Mon, 19 Apr 2021 12:49:08 -0700 (PDT) MIME-Version: 1.0 References: <20210419184516.GC1472665@redhat.com> In-Reply-To: <20210419184516.GC1472665@redhat.com> From: Dan Williams Date: Mon, 19 Apr 2021 12:48:58 -0700 Message-ID: Subject: Re: [PATCH][v2] dax: Fix missed wakeup during dax entry invalidation To: Vivek Goyal Cc: Linux fsdevel mailing list , Jan Kara , Matthew Wilcox , virtio-fs-list , Sergio Lopez , Miklos Szeredi , linux-nvdimm , linux kernel mailing list Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Apr 19, 2021 at 11:45 AM Vivek Goyal wrote: > > This is V2 of the patch. Posted V1 here. > > https://lore.kernel.org/linux-fsdevel/20210416173524.GA1379987@redhat.com/ > > Based on feedback from Dan and Jan, modified the patch to wake up > all waiters when dax entry is invalidated. This solves the issues > of missed wakeups. Care to send a formal patch with this commentary moved below the --- line? One style fixup below... > > 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 256M to reproduce > the problem consistently. > > 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. > > This patch fixes the issue by waking up all waiters when a dax entry > has been invalidated. This seems to fix the deadlock I am facing > and I can make forward progress. > > Reported-by: Sergio Lopez > Signed-off-by: Vivek Goyal > --- > fs/dax.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > Index: redhat-linux/fs/dax.c > =================================================================== > --- redhat-linux.orig/fs/dax.c 2021-04-16 14:16:44.332140543 -0400 > +++ redhat-linux/fs/dax.c 2021-04-19 11:24:11.465213474 -0400 > @@ -264,11 +264,11 @@ static void wait_entry_unlocked(struct x > finish_wait(wq, &ewait.wait); > } > > -static void put_unlocked_entry(struct xa_state *xas, void *entry) > +static void put_unlocked_entry(struct xa_state *xas, void *entry, bool wake_all) > { > /* If we were the only waiter woken, wake the next one */ > if (entry && !dax_is_conflict(entry)) > - dax_wake_entry(xas, entry, false); > + dax_wake_entry(xas, entry, wake_all); > } > > /* > @@ -622,7 +622,7 @@ struct page *dax_layout_busy_page_range( > entry = get_unlocked_entry(&xas, 0); > if (entry) > page = dax_busy_page(entry); > - put_unlocked_entry(&xas, entry); > + put_unlocked_entry(&xas, entry, false); I'm not a fan of raw true/false arguments because if you read this line in isolation you need to go read put_unlocked_entry() to recall what that argument means. So lets add something like: /** * enum dax_entry_wake_mode: waitqueue wakeup toggle * @WAKE_NEXT: entry was not mutated * @WAKE_ALL: entry was invalidated, or resized */ enum dax_entry_wake_mode { WAKE_NEXT, WAKE_ALL, } ...and use that as the arg for dax_wake_entry(). So I'd expect this to be a 3 patch series, introduce dax_entry_wake_mode for dax_wake_entry(), introduce the argument for put_unlocked_entry() without changing the logic, and finally this bug fix. Feel free to add 'Fixes: ac401cc78242 ("dax: New fault locking")' in case you feel this needs to be backported.