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 2E245C61DF4 for ; Fri, 24 Nov 2023 06:05:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230269AbjKXGEy (ORCPT ); Fri, 24 Nov 2023 01:04:54 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57056 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231255AbjKXGES (ORCPT ); Fri, 24 Nov 2023 01:04:18 -0500 Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [IPv6:2a03:a000:7:0:5054:ff:fe1c:15ff]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C9C0810C7; Thu, 23 Nov 2023 22:04:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-Id:Date:Subject:Cc:To:From: Reply-To:Content-Type:Content-ID:Content-Description; bh=RiZ+r8FbLExsvm//mT+YJVahGYKvclc7LYL6er4dEWE=; b=Y4JKQzTktLvOt/kY8qX7d2wYpF aPCfYY2W8xTpb7JRHL6ntOWeKF/D/j0gSif4/M30412UYwjBIg1RPtrvHTYF85pqQcRdbgvEskJuV R9t1vO0kt5Z8rrRQzMWi5SK/2EijTjDh0jAZOO3tVe7wL/v7JfrDgKnnQfsRQmqjsrZDaq+U49o6Q uHbh/VbtfF8Wi/e9pZ2WN06uytmlmMSY6WCagghs5mlZaTkJxa01na7UuzwkfEHZ3bruZF+U6Vb7f jgOhrZqdE/JlnTF+ZWuhB/ACyv1eURGEz/KzoNWpDUaPj6KsgC0MysZHtulE0+AWYsOadTAFdBsGW xAJihuKA==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1r6PId-002Pu3-1F; Fri, 24 Nov 2023 06:04:23 +0000 From: Al Viro To: linux-fsdevel@vger.kernel.org Cc: Linus Torvalds , Christian Brauner , linux-kernel@vger.kernel.org Subject: [PATCH v3 07/21] fast_dput(): handle underflows gracefully Date: Fri, 24 Nov 2023 06:04:08 +0000 Message-Id: <20231124060422.576198-7-viro@zeniv.linux.org.uk> X-Mailer: git-send-email 2.40.1 In-Reply-To: <20231124060422.576198-1-viro@zeniv.linux.org.uk> References: <20231124060200.GR38156@ZenIV> <20231124060422.576198-1-viro@zeniv.linux.org.uk> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If refcount is less than 1, we should just warn, unlock dentry and return true, so that the caller doesn't try to do anything else. Taking care of that leaves the rest of "lockref_put_return() has failed" case equivalent to "decrement refcount and rejoin the normal slow path after the point where we grab ->d_lock". NOTE: lockref_put_return() is strictly a fastpath thing - unlike the rest of lockref primitives, it does not contain a fallback. Caller (and it looks like fast_dput() is the only legitimate one in the entire kernel) has to do that itself. Reasons for lockref_put_return() failures: * ->d_lock held by somebody * refcount <= 0 * ... or an architecture not supporting lockref use of cmpxchg - sparc, anything non-SMP, config with spinlock debugging... We could add a fallback, but it would be a clumsy API - we'd have to distinguish between: (1) refcount > 1 - decremented, lock not held on return (2) refcount < 1 - left alone, probably no sense to hold the lock (3) refcount is 1, no cmphxcg - decremented, lock held on return (4) refcount is 1, cmphxcg supported - decremented, lock *NOT* held on return. We want to return with no lock held in case (4); that's the whole point of that thing. We very much do not want to have the fallback in case (3) return without a lock, since the caller might have to retake it in that case. So it wouldn't be more convenient than doing the fallback in the caller and it would be very easy to screw up, especially since the test coverage would suck - no way to test (3) and (4) on the same kernel build. Reviewed-by: Christian Brauner Signed-off-by: Al Viro --- fs/dcache.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index 00c19041adf3..9edabc7e2e64 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -779,12 +779,12 @@ static inline bool fast_dput(struct dentry *dentry) */ if (unlikely(ret < 0)) { spin_lock(&dentry->d_lock); - if (dentry->d_lockref.count > 1) { - dentry->d_lockref.count--; + if (WARN_ON_ONCE(dentry->d_lockref.count <= 0)) { spin_unlock(&dentry->d_lock); return true; } - return false; + dentry->d_lockref.count--; + goto locked; } /* @@ -842,6 +842,7 @@ static inline bool fast_dput(struct dentry *dentry) * else could have killed it and marked it dead. Either way, we * don't need to do anything else. */ +locked: if (dentry->d_lockref.count) { spin_unlock(&dentry->d_lock); return true; -- 2.39.2