From patchwork Wed Nov 1 06:20:51 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Al Viro X-Patchwork-Id: 13442541 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0AE574C67 for ; Wed, 1 Nov 2023 06:21:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linux.org.uk header.i=@linux.org.uk header.b="H6lAplmW" 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 2646BF7 for ; Tue, 31 Oct 2023 23:21:06 -0700 (PDT) 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=3I+Y9jWk7ME9G/i4K7RrRWTK8KeuuIOzpJCBsZA1NCM=; b=H6lAplmWd+JfFKUOiNYDxGmLPt BWJFq6UqsCKxCD4SBCHutW7Ez7czPd3luRwJrjseNw2jcDtfO3+cal5ktKb+V9bgOFo6QjDKu8x5w 2F1Ew1EtJ7Tl8tYVhT5dZl0nu/iNILQRRbeFcQmEu8Ib4uV2k2Fimmc1hasZ2uBS4dsqI8SrlYEE0 YnWxsKLgKjqWXxZ9Y9CwwigauOg1aVM51iPTQLEAdV+8JSNb7uIVugyd4CTnYnoT+qh8x+7sjzMeJ KcUlXLlFummG3UWvy1Bihl3ELyABI717tdLMPMSwO69gO53bRLYUejdTThlgjGVbdx4dQIbJCNcNK 0gCWkH5Q==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1qy4bA-008pb0-2G; Wed, 01 Nov 2023 06:21:04 +0000 From: Al Viro To: Linus Torvalds Cc: linux-fsdevel@vger.kernel.org Subject: [PATCH 02/15] fast_dput(): handle underflows gracefully Date: Wed, 1 Nov 2023 06:20:51 +0000 Message-Id: <20231101062104.2104951-2-viro@zeniv.linux.org.uk> X-Mailer: git-send-email 2.40.1 In-Reply-To: <20231101062104.2104951-1-viro@zeniv.linux.org.uk> References: <20231031061226.GC1957730@ZenIV> <20231101062104.2104951-1-viro@zeniv.linux.org.uk> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Sender: Al Viro 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. 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 5ec14df04f11..ddc534b39c22 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;