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Date:   Tue, 3 Jul 2018 00:35:25 +0000
From:   Christopher Lameter <cl@linux.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Andy Lutomirski <luto@amacapital.net>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
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        Russell King <linux@arm.linux.org.uk>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andi Kleen <andi@firstfloor.org>,
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Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate
 user inputs
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On Mon, 2 Jul 2018, Mathieu Desnoyers wrote:

> >
> > Platforms with 32 bit word size only guarantee atomicity of a 32 bit
> > write or RMV instruction.
> >
> > Special instructions may exist on a platform to perform 64 bit atomic
> > updates. We use cmpxchg64 f.e. on Intel 32 bit platforms to guarantee
> > atomicity8.
> >
> > So use the macros that we have to guarantee 64 bit ops and you should be
> > fine. See linux/arch/x86/include/asm/atomic64_32.h
>
> We are talking about user-space here. What we need is a single instruction
> atomic store, similar to what WRITE_ONCE() does in the kernel. The discussion
> is about whether doing the user-space equivalent of a WRITE_ONCE() to a u64
> on a 32-bit architecture should be considered to provide single-copy atomicity
> on the low 32 bits.

Right. You would need to make this work for userspace. atomic64_32.h is a
good reference as to which instructions provide 64 bit atomicity on 32
bit platforms.