From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from firstgate.proxmox.com (firstgate.proxmox.com [212.224.123.68]) by lore.proxmox.com (Postfix) with ESMTPS id DBDA21FF165 for ; Thu, 6 Nov 2025 14:56:13 +0100 (CET) Received: from firstgate.proxmox.com (localhost [127.0.0.1]) by firstgate.proxmox.com (Proxmox) with ESMTP id 8D5A917E49; Thu, 6 Nov 2025 14:56:54 +0100 (CET) Date: Thu, 06 Nov 2025 14:56:48 +0100 From: Fabian =?iso-8859-1?q?Gr=FCnbichler?= To: Proxmox Backup Server development discussion References: <20251106125458.479328-1-c.ebner@proxmox.com> In-Reply-To: <20251106125458.479328-1-c.ebner@proxmox.com> MIME-Version: 1.0 User-Agent: astroid/0.17.0 (https://github.com/astroidmail/astroid) Message-Id: <1762436861.ipo8b4a3lk.astroid@yuna.none> X-Bm-Milter-Handled: 55990f41-d878-4baa-be0a-ee34c49e34d2 X-Bm-Transport-Timestamp: 1762437392139 X-SPAM-LEVEL: Spam detection results: 0 AWL 0.048 Adjusted score from AWL reputation of From: address BAYES_00 -1.9 Bayes spam probability is 0 to 1% DMARC_MISSING 0.1 Missing DMARC policy KAM_DMARC_STATUS 0.01 Test Rule for DKIM or SPF Failure with Strict Alignment SPF_HELO_NONE 0.001 SPF: HELO does not publish an SPF Record SPF_PASS -0.001 SPF: sender matches SPF record URIBL_BLOCKED 0.001 ADMINISTRATOR NOTICE: The query to URIBL was blocked. See http://wiki.apache.org/spamassassin/DnsBlocklists#dnsbl-block for more information. [proxmox.com] Subject: Re: [pbs-devel] [PATCH proxmox-backup] chunk store: fix race window between chunk stat and gc cleanup X-BeenThere: pbs-devel@lists.proxmox.com X-Mailman-Version: 2.1.29 Precedence: list List-Id: Proxmox Backup Server development discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Proxmox Backup Server development discussion Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: pbs-devel-bounces@lists.proxmox.com Sender: "pbs-devel" On November 6, 2025 1:54 pm, Christian Ebner wrote: > Sweeping of unused chunks during garbage collection checks their > atime to distinguish between chunks being in-use and chunks no > longer being used. While garbage collection does lock the chunk > store by guarding its mutex before reading file stats and deleting > unused chunks, the conditional touch did not do this before updating > the chunks atime (thereby also checking the presence). > > Therefore there is a race window between the chunks metadata being > read and the chunk being removed, but the chunk being touched > in-between. > > The race is however rare, as for this to happen the chunk must be > older than the cutoff time and not be referenced by any index file, > otherwise the atime would be updated during phase 1 already. > > Fix by guarding the chunk store mutex before touching a chunk. > > Signed-off-by: Christian Ebner > --- > pbs-datastore/src/chunk_store.rs | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/pbs-datastore/src/chunk_store.rs b/pbs-datastore/src/chunk_store.rs > index ba7618e40..d21db4a71 100644 > --- a/pbs-datastore/src/chunk_store.rs > +++ b/pbs-datastore/src/chunk_store.rs > @@ -217,6 +217,7 @@ impl ChunkStore { > assert!(self.locker.is_some()); > > let (chunk_path, _digest_str) = self.chunk_path(digest); > + let _lock = self.mutex.lock(); > self.cond_touch_path(&chunk_path, assert_exists) alas, it's not as simple as that - this helper is also called while already holding the mutex, so we need to split it up further else we deadlock immediately on chunk insertion.. 1. make the existing cond_touch_chunk private and give it _no_lock suffix 2. make touch_chunk private and make it call the _no_lock variant 3. add a new cond_touch_chunk helper that obtains the lock and calls _no_lock internally 4. analyze other callers to ensure nobody else calls us with the mutex held already and while looking at that, I realized that index_mark_used_chunks is creating a chunk marker without holding a lock. but alas, that could (would) then be solved with your chunk-flock series, since it's only in the S3 case.. > } > > -- > 2.47.3 > > > > _______________________________________________ > pbs-devel mailing list > pbs-devel@lists.proxmox.com > https://lists.proxmox.com/cgi-bin/mailman/listinfo/pbs-devel > > > _______________________________________________ pbs-devel mailing list pbs-devel@lists.proxmox.com https://lists.proxmox.com/cgi-bin/mailman/listinfo/pbs-devel