From nobody Fri Oct 3 23:08:25 2025 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 1EBBF2E88B3 for ; Sat, 23 Aug 2025 16:39:14 +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=1755967156; cv=none; b=A+srVXOCgvlOLZvwER35g5oKStDdkBRHcOG3wij/vwTlvcg082fQ0ackSGuCVrPKZu5wAOshdrzUpu5AB9vFU5EfLAZloqFvV3Tgw8O3NNLxfb4ltIAB4m9FVK3I7WfurKDLiAz9tzpR5/SVWnizsOpqGS0Xl6SGMbbHksbTSwY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755967156; c=relaxed/simple; bh=R8ZMatXw1QnJnKgh1mnBmeMcfszM5opiigapcj/pbAM=; h=Message-ID:From:To:Cc:Subject:References:MIME-Version: Content-Type:Date; b=og0sta8dm1O2sIGq/9bNOsd6icnQ7hmNgMkv+j7XlwwIcU3OOEQfNQdVgG5WGRE0iJDqxzIcbVtwujWC1jHaLfi42qH716WWfef41mWlyVyBZDI3jDpAHzitokpHYK/EBxYqAoE2fDxd/UFyMs0yov7/xKENBLxYMRYpt74l5pY= 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=APZ8IoYH; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=Mnrk6zx8; 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="APZ8IoYH"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="Mnrk6zx8" Message-ID: <20250823161653.452303254@linutronix.de> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1755967153; 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=RLR8JIghH49Miso1nJUJEAbkKfMAqqR2bK/RmQbruPo=; b=APZ8IoYHQjqmRehvwD4cxQB/JBIMLy/9BYq/ucRsWgyowPialQWLz0sRdKSR5/JHapAGzr OMJvj1dQI84RuAWmnbCs1KMzgOsuf2BJIFu+jDP6eSiLdBhqulTFrR58vDzGxmVUF6FcWF ucGIV+7qx94sMJwBojCYhKEf9JHGSyCR53DA+hcBvvF/ahmLl+yAomXclXtn3fj/WmeWb8 FSxDOAQSUaJh2dgBkgP9odQZvgZi/UR3zwqvHiW7yE4io7OAjiody2aBeJw9nwgcpPDVng VN1vRtcYSp3C4kiF9yT/pl++EkU9b/uhmPa/0kqCxaWs2n3Y9UPyT0tH5nr10w== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1755967153; 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=RLR8JIghH49Miso1nJUJEAbkKfMAqqR2bK/RmQbruPo=; b=Mnrk6zx845WSGZ1PXD1U7kFHbfVpM/jdp8S+OJP3qC9CeSgSKVWpOwBH52keVKEsUrPSqJ PE0Noz65jlXtfXBQ== From: Thomas Gleixner To: LKML Cc: 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 V2 03/37] resq: Move algorithm comment to top References: <20250823161326.635281786@linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Date: Sat, 23 Aug 2025 18:39:12 +0200 (CEST) Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 */ =20 +/* + * 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 =3D TLS->rseq::cpu_id_start + * [1] TLS->rseq::rseq_cs =3D rseq_cs + * [start_ip] ---------------------------- + * [2] if (cpu !=3D 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] >=3D [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 =20 -/* - * - * 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 =3D TLS->rseq::cpu_id_start - * [1] TLS->rseq::rseq_cs =3D rseq_cs - * [start_ip] ---------------------------- - * [2] if (cpu !=3D 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] >=3D [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 =3D t->rseq;