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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 4 Jan 2018 14:23:25 -0800
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Subject: Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier
To: Dan Williams <dan.j.williams@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>, Julia Lawall <julia.lawall@lip6.fr>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        linux-arch@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Elena Reshetova <elena.reshetova@intel.com>,
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On Thu, Jan 4, 2018 at 2:20 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> #define array_access(base, idx, max) ({                         \
>         union { typeof(base[0]) _val; unsigned long _bit; } __u;\
>         unsigned long _i = (idx);                               \
>         unsigned long _m = (max);                               \
>         unsigned long _mask = _i < _m ? ~0 : 0;                 \
>         OPTIMIZER_HIDE_VAR(_mask);                              \
>         __u._val = base[_i & _mask];                            \
>         __u._bit &= _mask;                                      \
>         __u._val; })

That

    __u._val = base[_i & _mask];

thing would have to be READ_ONCE() to make it all ok for the special
cases without locking etc.

              Linus