From: "Arnd Bergmann" <arnd@arndb.de>
To: "Peter Zijlstra" <peterz@infradead.org>
Cc: "Thomas Gleixner" <tglx@linutronix.de>,
"Jens Axboe" <axboe@kernel.dk>,
linux-kernel@vger.kernel.org, "Ingo Molnar" <mingo@redhat.com>,
"Darren Hart" <dvhart@infradead.org>,
dave@stgolabs.net, andrealmeid@igalia.com,
"Andrew Morton" <akpm@linux-foundation.org>,
urezki@gmail.com, "Christoph Hellwig" <hch@infradead.org>,
"Lorenzo Stoakes" <lstoakes@gmail.com>,
linux-api@vger.kernel.org, linux-mm@kvack.org,
Linux-Arch <linux-arch@vger.kernel.org>,
malteskarupke@web.de
Subject: Re: [RFC][PATCH 04/10] futex: Add sys_futex_wake()
Date: Fri, 14 Jul 2023 22:10:10 +0200 [thread overview]
Message-ID: <d8366484-63d7-4e7c-b02d-7354aa69c444@app.fastmail.com> (raw)
In-Reply-To: <20230714144749.GA3261758@hirez.programming.kicks-ass.net>
On Fri, Jul 14, 2023, at 16:47, Peter Zijlstra wrote:
> On Fri, Jul 14, 2023 at 04:26:45PM +0200, Arnd Bergmann wrote:
>> On Fri, Jul 14, 2023, at 15:39, Peter Zijlstra wrote:
>> >
>> > +++ b/include/linux/syscalls.h
>> > @@ -563,6 +563,9 @@ asmlinkage long sys_set_robust_list(stru
>> > asmlinkage long sys_futex_waitv(struct futex_waitv *waiters,
>> > unsigned int nr_futexes, unsigned int flags,
>> > struct __kernel_timespec __user *timeout, clockid_t clockid);
>> > +
>> > +asmlinkage long sys_futex_wake(void __user *uaddr, int nr, unsigned
>> > int flags, u64 mask);
>> > +
>>
>> You can't really use 'u64' arguments in portable syscalls, it causes
>> a couple of problems, both with defining the user space wrappers,
>> and with compat mode.
>>
>> Variants that would work include:
>>
>> - using 'unsigned long' instead of 'u64'
>> - passing 'mask' by reference, as in splice()
>> - passing the mask in two u32-bit arguments like in llseek()
>>
>> Not sure if any of the above work for you.
>
> Durr, I was hoping they'd use register pairs, but yeah I can see how
> that would be very hard to do in generic code.
It kind of works to just use register pairs, the actual problem
you run into here is that:
- depending on the architecture, the register pairs need to be
even/odd pairs, so there are two different ways that 32-bit
architectures handle it
- The compat handler needs to explicitly name the registers that
are used, so to make your version above work correctly, we'd
need three entry points, for native 64-bit, compat 32-bit
odd/even pairs and compat 32-bit even/odd pairs.
> Hurmph.. using 2 u32s is unfortunate on 64bit, while unsigned long
> would limit 64bit futexes to 64bit machines (perhaps not too bad).
>
> Using unsigned long would help with the futex_wait() thing as well.
>
> I'll ponder things a bit.
>
> Obviously I only did build x86_64 ;-)
I suspect that restricting the futexes to native work size is
ok since many 32-bit architectures don't have 64-bit atomic
instructions anyway (armv6k+ and i586tsc+ being the obvious
exceptions), so userspace code that relies on it becomes
nonportable.
Arnd
next prev parent reply other threads:[~2023-07-14 20:10 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-14 13:38 [RFC][PATCH 00/10] futex: More Futex2 bits Peter Zijlstra
2023-07-14 13:39 ` [RFC][PATCH 01/10] futex: Clarify FUTEX2 flags Peter Zijlstra
2023-07-14 13:39 ` [RFC][PATCH 02/10] futex: Extend the " Peter Zijlstra
2023-07-14 13:39 ` [RFC][PATCH 03/10] futex: Flag conversion Peter Zijlstra
2023-07-14 13:39 ` [RFC][PATCH 04/10] futex: Add sys_futex_wake() Peter Zijlstra
2023-07-14 14:26 ` Arnd Bergmann
2023-07-14 14:47 ` Peter Zijlstra
2023-07-14 20:10 ` Arnd Bergmann [this message]
2023-07-14 13:39 ` [RFC][PATCH 05/10] mm: Add vmalloc_huge_node() Peter Zijlstra
2023-07-14 14:37 ` Matthew Wilcox
2023-07-14 15:09 ` Peter Zijlstra
2023-07-14 15:11 ` Matthew Wilcox
2023-07-14 15:26 ` Peter Zijlstra
2023-07-14 13:39 ` [RFC][PATCH 06/10] futex: Propagate flags into get_futex_key() Peter Zijlstra
2023-07-14 13:39 ` [RFC][PATCH 07/10] futex: Implement FUTEX2_NUMA Peter Zijlstra
2023-07-14 15:22 ` Peter Zijlstra
2023-07-14 13:39 ` [RFC][PATCH 08/10] futex: Propagate flags into futex_get_value_locked() Peter Zijlstra
2023-07-14 13:39 ` [RFC][PATCH 09/10] futex: Enable FUTEX2_{8,16} Peter Zijlstra
2023-07-14 13:39 ` [HACK][PATCH 10/10] futex: Munge size and numa into the legacy interface Peter Zijlstra
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