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From:   Alexander Lobakin <alexandr.lobakin@intel.com>
To:     Marco Elver <elver@google.com>
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        Tony Luck <tony.luck@intel.com>,
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Subject: Re: [PATCH v2 2/6] bitops: always define asm-generic non-atomic bitops
Date:   Mon, 13 Jun 2022 16:19:47 +0200
Message-Id: <20220613141947.1176100-1-alexandr.lobakin@intel.com>
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From: Marco Elver <elver@google.com>
Date: Fri, 10 Jun 2022 18:32:36 +0200

> On Fri, 10 Jun 2022 at 18:02, Luck, Tony <tony.luck@intel.com> wrote:
> >
> > > > +/**
> > > > + * generic_test_bit - Determine whether a bit is set
> > > > + * @nr: bit number to test
> > > > + * @addr: Address to start counting from
> > > > + */
> > >
> > > Shouldn't we add in this or in separate patch a big NOTE to explain that this
> > > is actually atomic and must be kept as a such?
> >
> > "atomic" isn't really the right word. The volatile access makes sure that the
> > compiler does the test at the point that the source code asked, and doesn't
> > move it before/after other operations.
> 
> It's listed in Documentation/atomic_bitops.txt.

Oh, so my memory was actually correct that I saw it in the docs
somewhere.
WDYT, should I mention this here in the code (block comment) as well
that it's atomic and must not lose `volatile` as Andy suggested or
it's sufficient to have it in the docs (+ it's not underscored)?

> 
> It is as "atomic" as READ_ONCE() or atomic_read() is. Though you are
> right that the "atomicity" of reading one bit is almost a given,
> because we can't really read half a bit.
> The main thing is that the compiler keeps it "atomic" and e.g. doesn't
> fuse the load with another or elide it completely, and then transforms
> the code in concurrency-unfriendly ways.
> 
> Like READ_ONCE() and friends, test_bit(), unlike non-atomic bitops,
> may also be used to dependency-order some subsequent marked (viz.
> atomic) operations.
> 
> > But there is no such thing as an atomic test_bit() operation:
> >
> >         if (test_bit(5, addr)) {
> >                 /* some other CPU nukes bit 5 */
> >
> >                 /* I know it was set when I looked, but now, could be anything */
> 
> The operation itself is atomic, because reading half a bit is
> impossible. Whether or not that bit is modified concurrently is a
> different problem.
> 
> Thanks,
> -- Marco

Thanks,
Olek