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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 962D6C25B0C for ; Mon, 8 Aug 2022 15:39:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243015AbiHHPjg (ORCPT ); Mon, 8 Aug 2022 11:39:36 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47254 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233652AbiHHPjf (ORCPT ); Mon, 8 Aug 2022 11:39:35 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3267A6599; Mon, 8 Aug 2022 08:39:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=XPl7JeNLjk9TMLNSixLvsZ+qQ4KPho8175mhT8bLnNk=; b=v4A/aroBq0LryFSV4/Iuj+CePP m/2j3fglEbks9mGRfNYBKV1iPkq4IN5rhOjpm0HbxzULHD5jnaFMgRAPHKslHdSoLpNu/EeMoSlPh Fiw8j16eHxehCM4fUs4fmZGWoIPT3QBwfDIb7mf4jTpi6wNzh8WeYqnPPA6Q2cobwzksPSMHlfISN qHg7OoKxEAGDU0SJwE2PYxHs17+jtZDyfXK8/VXUM29uMieBz7YRX7kUsh0R05Y+oJUMnWGeomNSL aWdkuecqof+I8HtG9vXkzgVxrpfVXl9vA4YCIsmMFWiHr6TKpDaD8qOtWmJ5gRH2VXxPQA3W5Nc9L GDJWprLA==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1oL4qY-00E1Vg-4F; Mon, 08 Aug 2022 15:39:14 +0000 Date: Mon, 8 Aug 2022 16:39:14 +0100 From: Matthew Wilcox To: Mikulas Patocka 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: Re: [PATCH v5] add barriers to buffer functions Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Aug 08, 2022 at 10:57:45AM -0400, Mikulas Patocka wrote: > On Mon, 8 Aug 2022, Matthew Wilcox wrote: > > > On Mon, Aug 08, 2022 at 10:26:10AM -0400, Mikulas Patocka wrote: > > > On Sun, 7 Aug 2022, Matthew Wilcox wrote: > > > > > +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 = 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; > > > > > +} > > > > > > > > Are there places that think that lock/unlock buffer implies a memory > > > > barrier? > > > > > > There's this in fs/reiserfs: > > > > > > if (!buffer_dirty(bh) && !buffer_locked(bh)) { > > > reiserfs_free_jh(bh); <--- this could be moved before buffer_locked > > > > It might be better to think of buffer_locked() as > > buffer_someone_has_exclusive_access(). I can't see the problem with > > moving the reads in reiserfs_free_jh() before the read of buffer_locked. > > > > > if (buffer_locked((journal->j_header_bh))) { > > > ... > > > } > > > journal->j_last_flush_trans_id = trans_id; > > > journal->j_first_unflushed_offset = offset; > > > jh = (struct reiserfs_journal_header *)(journal->j_header_bh->b_data); <--- this could be moved before buffer_locked > > > > I don't think b_data is going to be changed while someone else holds > > the buffer locked. That's initialised by set_bh_page(), which is an > > initialisation-time thing, before the BH is visible to any other thread. > > So, do you think that we don't need a barrier in buffer_locked()? That's my feeling. Of course, you might not be the only one confused, and if fs authors in general have made the mistake of thinking that buffer_locked is serialising, then it might be better to live up to that expectation. > There is also this (where the BUG_ON(!buffer_uptodate(bh)) saves it). > if (buffer_locked(bh)) { > int depth; > PROC_INFO_INC(sb, scan_bitmap.wait); > depth = reiserfs_write_unlock_nested(sb); > __wait_on_buffer(bh); > reiserfs_write_lock_nested(sb, depth); > } > BUG_ON(!buffer_uptodate(bh)); > BUG_ON(atomic_read(&bh->b_count) == 0); > > if (info->free_count == UINT_MAX) > reiserfs_cache_bitmap_metadata(sb, bh, info); <--- this could be moved before buffer_locked if there were no BUG_ONs It could be moved before buffer_locked(), but I don't see the harm in that. Look at how reiserfs_read_bitmap_block() gets the bh: bh = sb_bread(sb, block); __bread_gfp() has either already read the buffer (and it's uptodate), in which case it returns it. Or it calls __bread_slow() which will do the read and check uptodate before returning it. I wouldn't be surprised to find that this buffer_locked() check is actually dead code, but I have no desire to dive into reiserfs far enough to find out whether it's dead code or not.