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 D8572C43461 for ; Fri, 16 Apr 2021 19:56:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B38C1613D1 for ; Fri, 16 Apr 2021 19:56:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243916AbhDPT4t (ORCPT ); Fri, 16 Apr 2021 15:56:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60182 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243842AbhDPT4m (ORCPT ); Fri, 16 Apr 2021 15:56:42 -0400 Received: from mail-ej1-x631.google.com (mail-ej1-x631.google.com [IPv6:2a00:1450:4864:20::631]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7FC0DC061756 for ; Fri, 16 Apr 2021 12:56:17 -0700 (PDT) Received: by mail-ej1-x631.google.com with SMTP id l4so43756814ejc.10 for ; Fri, 16 Apr 2021 12:56:17 -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=l12BmPzjPGQPhOVAK1+pGnVg6YOohJn2PXow4I2KCwo=; b=nQnk4fUrtR73OTn0Bked0GsH77E4CU2Aryt7NrztyQzLtex0tRik3lEi7dybmcBzRH imUnySkDeWbLpm1SmLfQ939xsEAUlHIlrHZFouijtfVnEwwG/v4ZQfXaj+HSc8sr43j+ v+8FG6MhnCm6G5B8vHIVVyp4FpcXRY7PYCkB9DDvlM+eVLprbZBp++dJAbMAVMCt5SYF q/qG9Xpz56IlrdEJbFFzzSPDCCvk4OjPw1eqpC1um+tl/xs3BG0Rvpm0vl3JBc7b9mtP P0H2tUoAEcqxONVhFCzFwaQG92PxbLWuDF0PKMf+1CmUgDTeVky9AKb8XU42U9NOfxiW nrZA== 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=l12BmPzjPGQPhOVAK1+pGnVg6YOohJn2PXow4I2KCwo=; b=HJ6u+fw7lfVgWRm0OqL2SLyozmE+ikizu2Iia9W/IVLSr33R12DlOiRgyLDi0eaSWy tL4wf8tmYW4UauAQKIl4AYNvOKqkBBplCqqCx2WqqtCS7Yi13RJLlH/qvEyE0apZxB5o jLxX7y4FOoEVtE2P/0wEYfo8cgtRVmh86MAL+aCfymQyII6r3oWDeqy+lnsiMFXylWzg +J2BqVGu8EbgaN+Efj6d57XsSkujHj9blBhBkaOoZC0o/HkI9/iVDOP/Jo7+Hvcou/eu f/GsGhPFckBekROAIv7YbyJll7inXWhW19fCZMDPK/V5Tcsyby7w1BmPRugp1EIH/9Ac n+XQ== X-Gm-Message-State: AOAM532gu0XW8Xk2NhcxTFfw1oDPcmez2Z6hJ1XCsDmFoIPImGjSNcTH BLvWTy1BAuIRWTZnF36lSZOuIm1akIXC4IjFRI4dhg== X-Google-Smtp-Source: ABdhPJyQvyIq/sIJGlqAF2MfMI37oI2jAKdM4rT3dS8JIUJ3W5auOyWaYVKZc/Khipbuc/aofSL7uwOHc1OwxgnwwFo= X-Received: by 2002:a17:907:7631:: with SMTP id jy17mr9952113ejc.418.1618602976235; Fri, 16 Apr 2021 12:56:16 -0700 (PDT) MIME-Version: 1.0 References: <20210416173524.GA1379987@redhat.com> In-Reply-To: <20210416173524.GA1379987@redhat.com> From: Dan Williams Date: Fri, 16 Apr 2021 12:56:05 -0700 Message-ID: Subject: Re: [PATCH] dax: Fix missed wakeup in put_unlocked_entry() 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 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); How does this work if entry is NULL? dax_entry_waitqueue() will not know if it needs to adjust the index. I think the fix might be to specify that put_unlocked_entry() in the invalidate path needs to do a wake_up_all().