linux-kernel - Re: [PATCH RFC 1/1] futex: Create set_robust

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Message-Id: <bde852ec-8e2f-4957-9368-00d8e5a422c4@app.fastmail.com>
Date: Thu, 24 Oct 2024 15:57:44 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: André Almeida <andrealmeid@...lia.com>,
 "Thomas Gleixner" <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>,
 "Peter Zijlstra" <peterz@...radead.org>,
 "Darren Hart" <dvhart@...radead.org>, "Davidlohr Bueso" <dave@...olabs.net>,
 sonicadvance1@...il.com
Cc: linux-kernel@...r.kernel.org, kernel-dev@...lia.com,
 linux-api@...r.kernel.org
Subject: Re: [PATCH RFC 1/1] futex: Create set_robust_list2

On Thu, Oct 24, 2024, at 14:57, André Almeida wrote:
> This new syscall allows to set multiple list to the same process. There
> are two list types: 32 and 64 bit lists.
>
> It supports up to 10 lists per process (see ROBUST_LISTS_PER_TASK). The
> lists are dynamically allocated on demand, as part of a linked list.
> This is the proposed interface:
>
> 	long set_robust_list2(void *head, int index, unsigned int flags)
>
> Userspace can ask to set the head of a new list using (index = -1).
> Kernel will allocate a new list, place in the linked list and return the
> new index to userspace.
>
> Userspace can modify an existing head by using an index >= 0. If the
> requested list doesn't exist, an error is returned.
>
> Userspace cannot remove a robust list.
>
> For now, flag is for the list type:
>
> 	enum robust_list_type {
> 	 	ROBUST_LIST_32BIT,
> 		ROBUST_LIST_64BIT,
> 	 };
>
> Signed-off-by: André Almeida <andrealmeid@...lia.com>

Hi André,

I have no opinion on the syscall itself, but I'll comment on
the way you hook it up:

>  arch/arm/tools/syscall.tbl             |  1 +
>  arch/x86/entry/syscalls/syscall_64.tbl |  1 +

If we agree on the number, this should be added to all
architectures at the same time. In particular, when
you add it to 32-bit arm, it also needs to be in the
corresponding arch/arm64/tools/syscall_32.tbl for
compat mode.

>  include/uapi/asm-generic/unistd.h      |  5 +-

This reminds me that I need to send the patch to remove this
file, nothing should use it any more, though we still have
the copy in tools/include/uapi/asm-generic/unistd.h that
still gets referenced until the scripts are changed to
use the syscall.tbl format.

> +	if (unlikely(!list_empty(list2))) {
> +		list_for_each_entry_safe(curr, n, list2, list) {
> +			if (curr->head != NULL) {
> +				if (curr->list_type == ROBUST_LIST_64BIT)
> +					exit_robust_list(tsk, curr->head);
> +				else if (curr->list_type == ROBUST_LIST_32BIT)
> +					compat_exit_robust_list(tsk, curr->head);
> +				curr->head = NULL;
> +			}

This looks like the behavior of a 32-bit task using
ROBUST_LIST_64BIT is different on native 32-bit kernels
compared to running on compat mode.

Assuming we want them to behave the same way, did you intend
ROBUST_LIST_64BIT to refer to 64-bit pointers on 32-bit
tasks, or should they use normal word-size pointers?

     Arnd