From mboxrd@z Thu Jan 1 00:00:00 1970 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 591172EA15E for ; Thu, 4 Sep 2025 22:20:43 +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=1757024445; cv=none; b=ZfYvpMI2dlUc+ptghOGW2F+U8PJc7IJsFxD+lz3wjTiZu1r0r3ffbWjzrbaTcGucHkRo79MMWbuZuk2MPSKEGnmZLAFtBicLb3Cb3tjriCEfl0DtOhQpllxG8TGgWsKPyKdO1WWhu13Uhtt7OoabSpOYewHx+ADAv1UDnRngBX0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757024445; c=relaxed/simple; bh=1xSC5YtfTpwnmtwwGCFqEXBAHGQs7cR6itG5X0MRowQ=; h=Message-ID:From:To:Cc:Subject:References:MIME-Version: Content-Type:Date; b=H8sxmloVQlpqMeXUwExDMRzafi/Kc1EH0RyWX5HSH2q2RMsrYKMq0AJmsPWHr4JROW9OEV4rjMeeK8Bl+a5sFBYqH8G6F4EuzdfnhbMb3UTT9kVBRhQu1MnND9v4qKhplT3dKfqNBFJHJKsgDeev7L16cLmuQB0/uP567upvVCs= 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=V0/nDeoa; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=G80xdZNQ; 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="V0/nDeoa"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="G80xdZNQ" Message-ID: <20250904185719.325898019@linutronix.de> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1757024440; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: references:references; bh=pdn4t/2ynllKNIsn5zTbArAyv3rBjQUvYlpSz4wte3Q=; b=V0/nDeoaY6tziDYKd2WemACOGKEns5PfRSpRjGS+8UkwH/xU/mPXSciiO1LaO/yAwaAyM8 MlNZH9DQxAc8Yr5AQSXgM3Fi14nbnMFbgj/86+WjKSAKP2lJV1c1XzSSey2ElOHZtUyzEs 730STIkG6cY0qCntdHEggHNoH0bSgVXR306OZCjCd+3vmmeCzgxG6sj4oOSSkqGCXVIVdf iYbx3a8ZJcZj+iH7pEUMJfwuTSCH5xyd7jt32LEtLFECw8D6TGy5kGzmuBqDxp4agxte4m dxq0U8Zjp9qdKJDb4NTnlVIPHyenoexkuZj0diIpE/9oja3U0K2Hsy/BXQs9TA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1757024440; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: references:references; bh=pdn4t/2ynllKNIsn5zTbArAyv3rBjQUvYlpSz4wte3Q=; b=G80xdZNQRjOGCtWd2Lz62m1UBsZ8MWDmdRH4HHYGPxBK0MrcawNOJaUX6WfCVmH+2mVqay LM6/bW19ds2OS7DA== From: Thomas Gleixner To: LKML Cc: Michael Jeanson , Jens Axboe , Mathieu Desnoyers , Peter Zijlstra , "Paul E. McKenney" , Boqun Feng , Paolo Bonzini , Sean Christopherson , Wei Liu , Dexuan Cui , x86@kernel.org, Arnd Bergmann , Heiko Carstens , Christian Borntraeger , Sven Schnelle , Huacai Chen , Paul Walmsley , Palmer Dabbelt Subject: [patch V3 03/37] rseq: Move algorithm comment to top References: <20250904185336.943880027@linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Date: Fri, 5 Sep 2025 00:20:39 +0200 (CEST) Move the comment which documents the RSEQ algorithm to the top of the file, so it does not create horrible diffs later when the actual implementation is fed into the mincer. Signed-off-by: Thomas Gleixner Reviewed-by: Mathieu Desnoyers --- kernel/rseq.c | 119 ++++++++++++++++++++++++++++------------------------------ 1 file changed, 59 insertions(+), 60 deletions(-) --- a/kernel/rseq.c +++ b/kernel/rseq.c @@ -8,6 +8,65 @@ * Mathieu Desnoyers */ +/* + * Restartable sequences are a lightweight interface that allows + * user-level code to be executed atomically relative to scheduler + * preemption and signal delivery. Typically used for implementing + * per-cpu operations. + * + * It allows user-space to perform update operations on per-cpu data + * without requiring heavy-weight atomic operations. + * + * Detailed algorithm of rseq user-space assembly sequences: + * + * init(rseq_cs) + * cpu = TLS->rseq::cpu_id_start + * [1] TLS->rseq::rseq_cs = rseq_cs + * [start_ip] ---------------------------- + * [2] if (cpu != TLS->rseq::cpu_id) + * goto abort_ip; + * [3] + * [post_commit_ip] ---------------------------- + * + * The address of jump target abort_ip must be outside the critical + * region, i.e.: + * + * [abort_ip] < [start_ip] || [abort_ip] >= [post_commit_ip] + * + * Steps [2]-[3] (inclusive) need to be a sequence of instructions in + * userspace that can handle being interrupted between any of those + * instructions, and then resumed to the abort_ip. + * + * 1. Userspace stores the address of the struct rseq_cs assembly + * block descriptor into the rseq_cs field of the registered + * struct rseq TLS area. This update is performed through a single + * store within the inline assembly instruction sequence. + * [start_ip] + * + * 2. Userspace tests to check whether the current cpu_id field match + * the cpu number loaded before start_ip, branching to abort_ip + * in case of a mismatch. + * + * If the sequence is preempted or interrupted by a signal + * at or after start_ip and before post_commit_ip, then the kernel + * clears TLS->__rseq_abi::rseq_cs, and sets the user-space return + * ip to abort_ip before returning to user-space, so the preempted + * execution resumes at abort_ip. + * + * 3. Userspace critical section final instruction before + * post_commit_ip is the commit. The critical section is + * self-terminating. + * [post_commit_ip] + * + * 4. + * + * On failure at [2], or if interrupted by preempt or signal delivery + * between [1] and [3]: + * + * [abort_ip] + * F1. + */ + #include #include #include @@ -98,66 +157,6 @@ static int rseq_validate_ro_fields(struc unsafe_put_user(value, &t->rseq->field, error_label) #endif -/* - * - * Restartable sequences are a lightweight interface that allows - * user-level code to be executed atomically relative to scheduler - * preemption and signal delivery. Typically used for implementing - * per-cpu operations. - * - * It allows user-space to perform update operations on per-cpu data - * without requiring heavy-weight atomic operations. - * - * Detailed algorithm of rseq user-space assembly sequences: - * - * init(rseq_cs) - * cpu = TLS->rseq::cpu_id_start - * [1] TLS->rseq::rseq_cs = rseq_cs - * [start_ip] ---------------------------- - * [2] if (cpu != TLS->rseq::cpu_id) - * goto abort_ip; - * [3] - * [post_commit_ip] ---------------------------- - * - * The address of jump target abort_ip must be outside the critical - * region, i.e.: - * - * [abort_ip] < [start_ip] || [abort_ip] >= [post_commit_ip] - * - * Steps [2]-[3] (inclusive) need to be a sequence of instructions in - * userspace that can handle being interrupted between any of those - * instructions, and then resumed to the abort_ip. - * - * 1. Userspace stores the address of the struct rseq_cs assembly - * block descriptor into the rseq_cs field of the registered - * struct rseq TLS area. This update is performed through a single - * store within the inline assembly instruction sequence. - * [start_ip] - * - * 2. Userspace tests to check whether the current cpu_id field match - * the cpu number loaded before start_ip, branching to abort_ip - * in case of a mismatch. - * - * If the sequence is preempted or interrupted by a signal - * at or after start_ip and before post_commit_ip, then the kernel - * clears TLS->__rseq_abi::rseq_cs, and sets the user-space return - * ip to abort_ip before returning to user-space, so the preempted - * execution resumes at abort_ip. - * - * 3. Userspace critical section final instruction before - * post_commit_ip is the commit. The critical section is - * self-terminating. - * [post_commit_ip] - * - * 4. - * - * On failure at [2], or if interrupted by preempt or signal delivery - * between [1] and [3]: - * - * [abort_ip] - * F1. - */ - static int rseq_update_cpu_node_id(struct task_struct *t) { struct rseq __user *rseq = t->rseq;