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Fri, 22 Mar 2024 19:28:23 +0000 (UTC) Received: from dovecot-director2.suse.de ([10.150.64.162]) by imap2.dmz-prg2.suse.org with ESMTPSA id Uc5sMdfb/WV5KwAAn2gu4w (envelope-from ); Fri, 22 Mar 2024 19:28:23 +0000 Date: Fri, 22 Mar 2024 20:21:08 +0100 From: David Sterba To: Tavian Barnes Cc: linux-btrfs@vger.kernel.org, Christoph Hellwig , Qu Wenruo , Chris Mason , Josef Bacik , David Sterba , linux-kernel@vger.kernel.org Subject: Re: [PATCH] btrfs: Fix race in read_extent_buffer_pages() Message-ID: <20240322192108.GK14596@twin.jikos.cz> Reply-To: dsterba@suse.cz References: <1ca6e688950ee82b1526bb3098852af99b75e6ba.1710551459.git.tavianator@tavianator.com> Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1ca6e688950ee82b1526bb3098852af99b75e6ba.1710551459.git.tavianator@tavianator.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) X-Spam-Level: Authentication-Results: smtp-out1.suse.de; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b="iEAR/L4U"; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=rNQIK5np X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Spamd-Result: default: False [-4.21 / 50.00]; HAS_REPLYTO(0.30)[dsterba@suse.cz]; RCVD_VIA_SMTP_AUTH(0.00)[]; R_DKIM_ALLOW(-0.20)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; ARC_NA(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; MIME_GOOD(-0.10)[text/plain]; REPLYTO_ADDR_EQ_FROM(0.00)[]; RCVD_COUNT_THREE(0.00)[3]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; MX_GOOD(-0.01)[]; DKIM_TRACE(0.00)[suse.cz:+]; RCPT_COUNT_SEVEN(0.00)[8]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.cz:dkim]; FUZZY_BLOCKED(0.00)[rspamd.com]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; NEURAL_HAM_SHORT(-0.20)[-1.000]; RCVD_TLS_ALL(0.00)[]; BAYES_HAM(-3.00)[100.00%] X-Spam-Score: -4.21 X-Rspamd-Queue-Id: ED5EA38917 X-Spam-Flag: NO On Fri, Mar 15, 2024 at 09:14:29PM -0400, Tavian Barnes wrote: > To prevent concurrent reads for the same extent buffer, > read_extent_buffer_pages() performs these checks: > > /* (1) */ > if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags)) > return 0; > > /* (2) */ > if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags)) > goto done; > > At this point, it seems safe to start the actual read operation. Once > that completes, end_bbio_meta_read() does > > /* (3) */ > set_extent_buffer_uptodate(eb); > > /* (4) */ > clear_bit(EXTENT_BUFFER_READING, &eb->bflags); > > Normally, this is enough to ensure only one read happens, and all other > callers wait for it to finish before returning. Unfortunately, there is > a racey interleaving: > > Thread A | Thread B | Thread C > ---------+----------+--------- > (1) | | > | (1) | > (2) | | > (3) | | > (4) | | > | (2) | > | | (1) > > When this happens, thread B kicks of an unnecessary read. Worse, thread > C will see UPTODATE set and return immediately, while the read from > thread B is still in progress. This race could result in tree-checker > errors like this as the extent buffer is concurrently modified: > > BTRFS critical (device dm-0): corrupted node, root=256 > block=8550954455682405139 owner mismatch, have 11858205567642294356 > expect [256, 18446744073709551360] > > Fix it by testing UPTODATE again after setting the READING bit, and if > it's been set, skip the unnecessary read. > > Fixes: d7172f52e993 ("btrfs: use per-buffer locking for extent_buffer reading") > Link: https://lore.kernel.org/linux-btrfs/CAHk-=whNdMaN9ntZ47XRKP6DBes2E5w7fi-0U3H2+PS18p+Pzw@mail.gmail.com/ > Link: https://lore.kernel.org/linux-btrfs/f51a6d5d7432455a6a858d51b49ecac183e0bbc9.1706312914.git.wqu@suse.com/ > Link: https://lore.kernel.org/linux-btrfs/c7241ea4-fcc6-48d2-98c8-b5ea790d6c89@gmx.com/ > Signed-off-by: Tavian Barnes Thank you very much for taking the time to debug the issue and for the fix. It is a rare occurrence that a tough bug is followed by a fix from the same person (outside of the developer group) and is certainly appreciated.