From nobody Sat Apr 11 20:14:04 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 536C3C19F2A for ; Sun, 7 Aug 2022 11:37:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233590AbiHGLhh (ORCPT ); Sun, 7 Aug 2022 07:37:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57712 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232726AbiHGLhe (ORCPT ); Sun, 7 Aug 2022 07:37:34 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 86E8C6560 for ; Sun, 7 Aug 2022 04:37:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1659872252; 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=o1EMRzMlY6/eJQLdXyI2BRTqKZiVN5D0fmDyLdtZOXU=; b=b3JThSPMkHHXNsOZY7tBUZWHAbPuWnB2y2wGKmL/FMUXUKA1DrE84clZvabCY5s3MIU+i+ 46kATKNobflH3so7n3QGsf1HFq24N3msFoZielTvuNkCKBIuKPliq/D7Jg7dCUITRXYNGS nLCjp3IZ/U70utGxmbAVLIowHGN7U50= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-653-BMw6k7YDOjC-6Vds6LUy8g-1; Sun, 07 Aug 2022 07:37:27 -0400 X-MC-Unique: BMw6k7YDOjC-6Vds6LUy8g-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 1588F3C021AD; Sun, 7 Aug 2022 11:37:26 +0000 (UTC) Received: from file01.intranet.prod.int.rdu2.redhat.com (file01.intranet.prod.int.rdu2.redhat.com [10.11.5.7]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E3AFB403D0E2; Sun, 7 Aug 2022 11:37:24 +0000 (UTC) Received: from file01.intranet.prod.int.rdu2.redhat.com (localhost [127.0.0.1]) by file01.intranet.prod.int.rdu2.redhat.com (8.14.4/8.14.4) with ESMTP id 277BbOAm030983; Sun, 7 Aug 2022 07:37:24 -0400 Received: from localhost (mpatocka@localhost) by file01.intranet.prod.int.rdu2.redhat.com (8.14.4/8.14.4/Submit) with ESMTP id 277BbMZf030978; Sun, 7 Aug 2022 07:37:23 -0400 X-Authentication-Warning: file01.intranet.prod.int.rdu2.redhat.com: mpatocka owned process doing -bs Date: Sun, 7 Aug 2022 07:37:22 -0400 (EDT) From: Mikulas Patocka X-X-Sender: mpatocka@file01.intranet.prod.int.rdu2.redhat.com To: Matthew Wilcox cc: Linus Torvalds , Will Deacon , "Paul E. McKenney" , Ard Biesheuvel , Alexander Viro , Alan Stern , Andrea Parri , Peter Zijlstra , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Daniel Lustig , Joel Fernandes , Linux Kernel Mailing List , linux-arch , linux-fsdevel@vger.kernel.org Subject: [PATCH v5] add barriers to buffer functions In-Reply-To: Message-ID: References: User-Agent: Alpine 2.02 (LRH 1266 2009-07-14) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: TEXT/PLAIN; charset="utf-8" On Fri, 5 Aug 2022, Matthew Wilcox wrote: > On Mon, Aug 01, 2022 at 11:01:40AM -0400, Mikulas Patocka wrote: > >=20 > > In most cases, the buffer is set uptodate while it is locked, so that=20 > > there is no race on the uptodate flag (the race exists on the locked=20 > > flag). Are there any cases where the uptodate flag is modified on unloc= ked=20 > > buffer, so that it needs special treatment too? >=20 > I think you misunderstand the purpose of locked/uptodate. At least > for pages, the lock flag does not order access to the data in the page. > Indeed, the contents of the page can be changed while you hold the lock. > But the uptodate flag does order access to the data. At the point where > you can observe the uptodate flag set, you know the contents of the page > have been completely read from storage. And you don't need to hold the > lock to check the uptodate flag. So this is wrong: >=20 > buffer_lock() > *data =3D 0x12345678; > buffer_set_uptodate_not_ordered() > buffer_unlock_ordered() >=20 > because a reader can do: >=20 > while (!buffer_test_uptodate()) { > buffer_lock(); > buffer_unlock(); > } > x =3D *data; >=20 > and get x !=3D 0x12345678 because the compiler can move the > buffer_set_uptodate_not_ordered() before the store to *data. Thanks for explanation. Would you like this patch? From: Mikulas Patocka Let's have a look at this piece of code in __bread_slow: get_bh(bh); bh->b_end_io =3D end_buffer_read_sync; submit_bh(REQ_OP_READ, 0, bh); wait_on_buffer(bh); if (buffer_uptodate(bh)) return bh; Neither wait_on_buffer nor buffer_uptodate contain any memory barrier. Consequently, if someone calls sb_bread and then reads the buffer data, the read of buffer data may be executed before wait_on_buffer(bh) on architectures with weak memory ordering and it may return invalid data. Fix this bug by adding a write memory barrier to set_buffer_uptodate and a read memory barrier to buffer_uptodate (in the same way as folio_test_uptodate and folio_mark_uptodate). We also add a barrier to buffer_locked - it pairs with a barrier in unlock_buffer. Signed-off-by: Mikulas Patocka Cc: stable@vger.kernel.org Index: linux-2.6/include/linux/buffer_head.h =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D --- linux-2.6.orig/include/linux/buffer_head.h +++ linux-2.6/include/linux/buffer_head.h @@ -117,10 +117,8 @@ static __always_inline int test_clear_bu * of the form "mark_buffer_foo()". These are higher-level functions which * do something in addition to setting a b_state bit. */ -BUFFER_FNS(Uptodate, uptodate) BUFFER_FNS(Dirty, dirty) TAS_BUFFER_FNS(Dirty, dirty) -BUFFER_FNS(Lock, locked) BUFFER_FNS(Req, req) TAS_BUFFER_FNS(Req, req) BUFFER_FNS(Mapped, mapped) @@ -135,6 +133,49 @@ BUFFER_FNS(Meta, meta) BUFFER_FNS(Prio, prio) BUFFER_FNS(Defer_Completion, defer_completion) =20 +static __always_inline void set_buffer_uptodate(struct buffer_head *bh) +{ + /* + * make it consistent with folio_mark_uptodate + * pairs with smp_acquire__after_ctrl_dep in buffer_uptodate + */ + smp_wmb(); + set_bit(BH_Uptodate, &bh->b_state); +} + +static __always_inline void clear_buffer_uptodate(struct buffer_head *bh) +{ + clear_bit(BH_Uptodate, &bh->b_state); +} + +static __always_inline int buffer_uptodate(const struct buffer_head *bh) +{ + bool ret =3D test_bit(BH_Uptodate, &bh->b_state); + /* + * make it consistent with folio_test_uptodate + * pairs with smp_wmb in set_buffer_uptodate + */ + if (ret) + smp_acquire__after_ctrl_dep(); + return ret; +} + +static __always_inline void set_buffer_locked(struct buffer_head *bh) +{ + set_bit(BH_Lock, &bh->b_state); +} + +static __always_inline int buffer_locked(const struct buffer_head *bh) +{ + bool ret =3D test_bit(BH_Lock, &bh->b_state); + /* + * pairs with smp_mb__after_atomic in unlock_buffer + */ + if (!ret) + smp_acquire__after_ctrl_dep(); + return ret; +} + #define bh_offset(bh) ((unsigned long)(bh)->b_data & ~PAGE_MASK) =20 /* If we *know* page->private refers to buffer_heads */