From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 5F6BE69317; Wed, 21 Feb 2024 14:22:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1708525331; cv=none; b=jtzRWhzxX93uZts4o6RB6LS3/9FZ7hyS0wEkl0tQPS1WrvBILkrNt+ZzvURiQr4TKn3suq0/gmAEk/xkTMaZHhXMrPWs40aA8GE+eMkJ9JHsvcUVUhLdAmi9mVlJEsBQAxeEJh6jsgHOy9UWNoIc4qurJjWX+Q9f1sQzNK07B54= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1708525331; c=relaxed/simple; bh=KzhvFE2KTOLFgZfLHVE4p9BWrdlJRtJXbMGiUX7QvQI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=QvPy6z0KDh3nda7FWdYLSdhJrgKWrQjkuaRdshBH2oN83dQ7D9E6AgImP4whKjzJokgepS9IW61RsTrigcHZqd/Af9k05z3iUHMYg9CjXUkYnXZUhtFNXsVM+YIa3DNXLIgjqimoKL2nLtYBvniZraq4I91CqWieJVpic0aokSY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=K5EiClsW; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="K5EiClsW" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C294CC433F1; Wed, 21 Feb 2024 14:22:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1708525331; bh=KzhvFE2KTOLFgZfLHVE4p9BWrdlJRtJXbMGiUX7QvQI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=K5EiClsWmAXTmuFdtv/zpUs7+Xz2SG2ATD7csr7ludHVj+vR79qQnut8eTMvrz8lN 1wfiYe6wE6dpxH+WMNa9NOyEH95QjEeNm9nt309AefO8osuXZ1p4oyF8b77sZ9yBDp ueTCLTYtD7Ccg4aKj1hdWxEZTv3wjTeeZRlKyHyA= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Christian Brauner , Al Viro , Sasha Levin Subject: [PATCH 5.4 122/267] fast_dput(): handle underflows gracefully Date: Wed, 21 Feb 2024 14:07:43 +0100 Message-ID: <20240221125943.834105585@linuxfoundation.org> X-Mailer: git-send-email 2.43.2 In-Reply-To: <20240221125940.058369148@linuxfoundation.org> References: <20240221125940.058369148@linuxfoundation.org> User-Agent: quilt/0.67 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Al Viro [ Upstream commit 504e08cebe1d4e1efe25f915234f646e74a364a8 ] 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 Signed-off-by: Sasha Levin --- fs/dcache.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index b2a7f1765f0b..43864a276faa 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -740,12 +740,12 @@ 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; } /* @@ -796,6 +796,7 @@ 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; -- 2.43.0