linux-kernel - Re: [patch V2 03/37] resq: Move algorithm comment to top

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Message-ID: <a700356f-9e51-4889-bc21-8fde13f5e1a2@efficios.com>
Date: Mon, 25 Aug 2025 11:41:28 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
 <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, x86@...nel.org,
 Arnd Bergmann <arnd@...db.de>, Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Huacai Chen <chenhuacai@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V2 03/37] resq: Move algorithm comment to top

On 2025-08-23 12:39, Thomas Gleixner wrote:
> 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.

Typo in the subject:

resq -> rseq

> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Other than this nit:

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

> ---
>   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 <mathieu.desnoyers@...icios.com>
>    */
>   
> +/*
> + * 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]               <last_instruction_in_cs>
> + *   [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.  <success>
> + *
> + *   On failure at [2], or if interrupted by preempt or signal delivery
> + *   between [1] and [3]:
> + *
> + *       [abort_ip]
> + *   F1. <failure>
> + */
> +
>   #include <linux/sched.h>
>   #include <linux/uaccess.h>
>   #include <linux/syscalls.h>
> @@ -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]               <last_instruction_in_cs>
> - *   [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.  <success>
> - *
> - *   On failure at [2], or if interrupted by preempt or signal delivery
> - *   between [1] and [3]:
> - *
> - *       [abort_ip]
> - *   F1. <failure>
> - */
> -
>   static int rseq_update_cpu_node_id(struct task_struct *t)
>   {
>   	struct rseq __user *rseq = t->rseq;
> 


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com