From nobody Wed Oct 1 22:32:37 2025 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 A6A633074AA; Wed, 1 Oct 2025 14:48:42 +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=1759330122; cv=none; b=SknlEzXxg2NcZkqBM1+CG5FOxpPolKuXWFr953OdIxiXcHgBEu/tXFK/PPbk1ucTR0+4MyzdfPBxozVWOOhGGh+EoXqiFbyIHY9/voR77xPioxhjx4BJlz8mUXfyFjbcuEZZAOj4Uu7BN7F7ezNGiGcef3xmaKOkxkhr9pr8hRM= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759330122; c=relaxed/simple; bh=LScGsjhQ8gC4uQeaMP9aDvziuKzyoZfiTdtxTzt+6cM=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=jXfQ0k2DMF8N5BIGC1LmxrUS2wtJ6a4iCHPraxHTSXVgrc4OEa2g981JZtl277kufXTOIJH+fwC/VVoK0gRBIpg1nlx7z7WFYxbpwQj5Rzyr11Rv+7wGlNlLder/cyRwo5embATJOzq4CNgg4xXtnNeKiEf1R6ak1NtwM17gjUc= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=N0djdl6Y; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="N0djdl6Y" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0D0B0C4AF0C; Wed, 1 Oct 2025 14:48:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1759330122; bh=LScGsjhQ8gC4uQeaMP9aDvziuKzyoZfiTdtxTzt+6cM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=N0djdl6YiJv2vY+1kcyDfo7HXbOY+Pb88LsyZuWnKHI6kgfoBWs08cQyvh8dSQjPs mSbQTRgLMcZEAKMFPKOsgaIafXDDCLbCBXPwP7Eh7B/sCd5fML6sdeQxAnwKE7ZkMk G74P+wzipD+BhOCyT47mly4ECQHYoG55lkNOHGNxmQVvV3Io5u6xkaCFyqq6cwDsQb PeVGbM2y2By7VGNjOWF+f6I8qsw7ujUa6ylQR34NL45WS2vIMerKdZmnt1lsCcM8em EJ5+avfVgbFXnj9844ONMDS6kG2Mq3WNMi9t8/8+Tg5amAfD6XyEYDE8658v3+szXF 0Tlw4Q4tL8Z+g== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 8CF7CCE14DF; Wed, 1 Oct 2025 07:48:34 -0700 (PDT) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, "Paul E. McKenney" , Mathieu Desnoyers , Sebastian Andrzej Siewior , bpf@vger.kernel.org Subject: [PATCH v2 17/21] srcu: Require special srcu_struct define/init for SRCU-fast readers Date: Wed, 1 Oct 2025 07:48:28 -0700 Message-Id: <20251001144832.631770-17-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: <7fa58961-2dce-4e08-8174-1d1cc592210f@paulmck-laptop> References: <7fa58961-2dce-4e08-8174-1d1cc592210f@paulmck-laptop> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" This commit adds CONFIG_PROVE_RCU=3Dy checking to enforce the new rule that srcu_struct structures passed to srcu_read_lock_fast() and other SRCU-fast read-side markers be either initialized with init_srcu_struct_fast() on the one hand or defined using either DEFINE_SRCU_FAST() or DEFINE_STATIC_SRCU_FAST(). This will enable removal of the non-debug read-side checks from srcu_read_lock_fast() and friends, which on my laptop provides a 25% speedup (which admittedly amounts to about half a nanosecond, but when tracing fastpaths...) Signed-off-by: Paul E. McKenney Cc: Mathieu Desnoyers Cc: Steven Rostedt Cc: Sebastian Andrzej Siewior Cc: --- include/linux/srcu.h | 34 ++++++++++++++++++++++------------ kernel/rcu/srcutree.c | 1 + 2 files changed, 23 insertions(+), 12 deletions(-) diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 26de47820c58cd..2982b5a6930fa6 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -271,17 +271,26 @@ static inline int srcu_read_lock(struct srcu_struct *= ssp) __acquires(ssp) * @ssp: srcu_struct in which to register the new reader. * * Enter an SRCU read-side critical section, but for a light-weight - * smp_mb()-free reader. See srcu_read_lock() for more information. - * - * If srcu_read_lock_fast() is ever used on an srcu_struct structure, - * then none of the other flavors may be used, whether before, during, - * or after. Note that grace-period auto-expediting is disabled for _fast - * srcu_struct structures because auto-expedited grace periods invoke - * synchronize_rcu_expedited(), IPIs and all. - * - * Note that srcu_read_lock_fast() can be invoked only from those contexts - * where RCU is watching, that is, from contexts where it would be legal - * to invoke rcu_read_lock(). Otherwise, lockdep will complain. + * smp_mb()-free reader. See srcu_read_lock() for more information. This + * function is NMI-safe, in a manner similar to srcu_read_lock_nmisafe(). + * + * For srcu_read_lock_fast() to be used on an srcu_struct structure, + * that structure must have been defined using either DEFINE_SRCU_FAST() + * or DEFINE_STATIC_SRCU_FAST() on the one hand or initialized with + * init_srcu_struct_fast() on the other. Such an srcu_struct structure + * cannot be passed to any non-fast variant of srcu_read_{,un}lock() or + * srcu_{down,up}_read(). In kernels built with CONFIG_PROVE_RCU=3Dy, + * __srcu_check_read_flavor() will complain bitterly if you ignore this + * restriction. + * + * Grace-period auto-expediting is disabled for SRCU-fast srcu_struct + * structures because SRCU-fast expedited grace periods invoke + * synchronize_rcu_expedited(), IPIs and all. If you need expedited + * SRCU-fast grace periods, use synchronize_srcu_expedited(). + * + * The srcu_read_lock_fast() function can be invoked only from those + * contexts where RCU is watching, that is, from contexts where it would + * be legal to invoke rcu_read_lock(). Otherwise, lockdep will complain. */ static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_st= ruct *ssp) __acquires(ssp) { @@ -317,7 +326,8 @@ static inline struct srcu_ctr __percpu *srcu_read_lock_= fast_notrace(struct srcu_ * srcu_down_read() for more information. * * The same srcu_struct may be used concurrently by srcu_down_read_fast() - * and srcu_read_lock_fast(). + * and srcu_read_lock_fast(). However, the same definition/initialization + * requirements called out for srcu_read_lock_safe() apply. */ static inline struct srcu_ctr __percpu *srcu_down_read_fast(struct srcu_st= ruct *ssp) __acquires(ssp) { diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index c29203b23d1ad7..2f8aa280911e19 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -766,6 +766,7 @@ void __srcu_check_read_flavor(struct srcu_struct *ssp, = int read_flavor) WARN_ON_ONCE(ssp->srcu_reader_flavor && read_flavor !=3D ssp->srcu_reader= _flavor); WARN_ON_ONCE(old_read_flavor && ssp->srcu_reader_flavor && old_read_flavor !=3D ssp->srcu_reader_flavor); + WARN_ON_ONCE(read_flavor =3D=3D SRCU_READ_FLAVOR_FAST && !ssp->srcu_reade= r_flavor); if (!old_read_flavor) { old_read_flavor =3D cmpxchg(&sdp->srcu_reader_flavor, 0, read_flavor); if (!old_read_flavor) --=20 2.40.1