From nobody Mon Feb 9 02:14:33 2026 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (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 9E45128FF; Wed, 29 Jan 2025 14:33:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738161239; cv=none; b=ouynR1P9EslNVQutEfdRdFYRSQ8Wus1KJ+v8SERIy1ElXN4ijQWlzgFPsW+tT0VILGwC9Zbzs1Ks1cGFkKc2VBAUodnM22fi8cy8KGQG5u0oxtGYgm8oc/3I14VkGR7gakw0JHx1PwezgQx7d4xzqMQmjdi0DleY0vITrl8abgI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738161239; c=relaxed/simple; bh=L74M3KHDpSyNvgRcggxvwCNCHyblalcP/GD0WNrpB4c=; h=Date:From:To:Subject:Cc:MIME-Version:Message-ID:Content-Type; b=u2EgFd6fAABW9Ha8+lTwQj/hWIhTIIr3qaPN7Rr6su9Y5kM+D/qbSpUlTdKBkuxYsjC8udQLMpx6KGgRyUy4SEyls85l/OsdciQsdE4a02NzsXZ3Yfg0PjZ1Ss8CjK/L6+YL9YDXGrKpkCjxT7xo7E7Vs5FMdQCP2VixPvXXzy8= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=WU54s9+8; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=H7i/Dwdm; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="WU54s9+8"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="H7i/Dwdm" Date: Wed, 29 Jan 2025 14:33:51 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1738161234; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=qMezJWQbRkxWMxDma2qkK64yeogvHOEOwq2Yrl8QBTg=; b=WU54s9+8xYBFm4+L4ZrSkJwOhOoUg8gh9rNw57YsKPMqETJa7ngcnyoUXBhgET4uaoe4by WJR4Nlf338BiE06peW9HreqtXKw1Qoo5O0P/hsqzuPHaKMmlFoPeE2yhDOOse/r0lYKJrG GFM+alVyRq/suunMpcVT653y1wuCZ/bHm1eq4FykNkEpYfU0BDoZrbqAf5o9GClv8EsCqo Gx3F7AtwOBFY3Xk66cc2/ymAtQ8NHwSvgS+qfcEUhAGkbL3z/BC9c0Peo1NhDOcEotX2O7 UBRuVBwOQUIYsyUvdb9NDU1hg8AWsD1QcdPQR5A78PjGVZKwkRWj9k/j/Xy+YA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1738161234; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=qMezJWQbRkxWMxDma2qkK64yeogvHOEOwq2Yrl8QBTg=; b=H7i/DwdmGQYV/EkQwR/GZGN2M+mjVej2i/6h/OcckZdfUUjtKpX4bG3E9dcrj/n3EYKtuW qm3Q6KnPry91hQCw== From: "tip-bot2 for Thomas Gleixner" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: locking/urgent] rcuref: Plug slowpath race in rcuref_put() Cc: kernel test robot , Thomas Gleixner , Sebastian Andrzej Siewior , stable@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Message-ID: <173816123142.31546.2255602054894403680.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Precedence: bulk Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable The following commit has been merged into the locking/urgent branch of tip: Commit-ID: b9a49520679e98700d3d89689cc91c08a1c88c1d Gitweb: https://git.kernel.org/tip/b9a49520679e98700d3d89689cc91c08a= 1c88c1d Author: Thomas Gleixner AuthorDate: Sun, 19 Jan 2025 00:55:32 +01:00 Committer: Thomas Gleixner CommitterDate: Wed, 29 Jan 2025 15:21:31 +01:00 rcuref: Plug slowpath race in rcuref_put() Kernel test robot reported an "imbalanced put" in the rcuref_put() slow path, which turned out to be a false positive. Consider the following race: ref =3D 0 (via rcuref_init(ref, 1)) T1 T2 rcuref_put(ref) -> atomic_add_negative_release(-1, ref) = # ref -> 0xffffffff -> rcuref_put_slowpath(ref) rcuref_get(ref) -> atomic_add_negative_relaxed(1, = &ref->refcnt) -> return true; = # ref -> 0 rcuref_put(ref) -> atomic_add_negative_release(-1,= ref) # ref -> 0xffffffff -> rcuref_put_slowpath() -> cnt =3D atomic_read(&ref->refcnt); = # cnt -> 0xffffffff / RCUREF_NOREF -> atomic_try_cmpxchg_release(&ref->refcnt, &cnt, RCUREF_DEAD)) = # ref -> 0xe0000000 / RCUREF_DEAD -> return true -> cnt =3D atomic_read(&ref->ref= cnt); # cnt -> 0xe0000000 / RCUREF_DEAD -> if (cnt > RCUREF_RELEASED) = # 0xe0000000 > 0xc0000000 -> WARN_ONCE(cnt >=3D RCUREF_R= ELEASED, "rcuref - imbalanced put()") The problem is the additional read in the slow path (after it decremented to RCUREF_NOREF) which can happen after the counter has been marked RCUREF_DEAD. Prevent this by reusing the return value of the decrement. Now every "final" put uses RCUREF_NOREF in the slow path and attempts the final cmpxchg() to RCUREF_DEAD. [ bigeasy: Add changelog ] Fixes: ee1ee6db07795 ("atomics: Provide rcuref - scalable reference countin= g") Reported-by: kernel test robot Debugged-by: Sebastian Andrzej Siewior Signed-off-by: Thomas Gleixner Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Thomas Gleixner Reviewed-by: Sebastian Andrzej Siewior Cc: stable@vger.kernel.org Closes: https://lore.kernel.org/oe-lkp/202412311453.9d7636a2-lkp@intel.com --- include/linux/rcuref.h | 9 ++++++--- lib/rcuref.c | 5 ++--- 2 files changed, 8 insertions(+), 6 deletions(-) diff --git a/include/linux/rcuref.h b/include/linux/rcuref.h index 2c8bfd0..6322d8c 100644 --- a/include/linux/rcuref.h +++ b/include/linux/rcuref.h @@ -71,27 +71,30 @@ static inline __must_check bool rcuref_get(rcuref_t *re= f) return rcuref_get_slowpath(ref); } =20 -extern __must_check bool rcuref_put_slowpath(rcuref_t *ref); +extern __must_check bool rcuref_put_slowpath(rcuref_t *ref, unsigned int c= nt); =20 /* * Internal helper. Do not invoke directly. */ static __always_inline __must_check bool __rcuref_put(rcuref_t *ref) { + int cnt; + RCU_LOCKDEP_WARN(!rcu_read_lock_held() && preemptible(), "suspicious rcuref_put_rcusafe() usage"); /* * Unconditionally decrease the reference count. The saturation and * dead zones provide enough tolerance for this. */ - if (likely(!atomic_add_negative_release(-1, &ref->refcnt))) + cnt =3D atomic_sub_return_release(1, &ref->refcnt); + if (likely(cnt >=3D 0)) return false; =20 /* * Handle the last reference drop and cases inside the saturation * and dead zones. */ - return rcuref_put_slowpath(ref); + return rcuref_put_slowpath(ref, cnt); } =20 /** diff --git a/lib/rcuref.c b/lib/rcuref.c index 97f300e..5bd726b 100644 --- a/lib/rcuref.c +++ b/lib/rcuref.c @@ -220,6 +220,7 @@ EXPORT_SYMBOL_GPL(rcuref_get_slowpath); /** * rcuref_put_slowpath - Slowpath of __rcuref_put() * @ref: Pointer to the reference count + * @cnt: The resulting value of the fastpath decrement * * Invoked when the reference count is outside of the valid zone. * @@ -233,10 +234,8 @@ EXPORT_SYMBOL_GPL(rcuref_get_slowpath); * with a concurrent get()/put() pair. Caller is not allowed to * deconstruct the protected object. */ -bool rcuref_put_slowpath(rcuref_t *ref) +bool rcuref_put_slowpath(rcuref_t *ref, unsigned int cnt) { - unsigned int cnt =3D atomic_read(&ref->refcnt); - /* Did this drop the last reference? */ if (likely(cnt =3D=3D RCUREF_NOREF)) { /*